Accessibility is a generic term that can be applied to several different situations, particularly:

• The ease or difficulty for disabled people to access the resources of Internet, computers or telecommunication systems.
• The ability to access a resource independently of the way to do it. For example a web site that can be accessed through Internet, mobile phones and PDAs and has a newsletter edited in paper has a great accessibility.

• Representing causality in an effective way.
• The acts showing communication or flow.
• Reorganisation or restructuring which adapts well to animation, whenever the complexity of the interaction isn't very high.
• Sequences of complex spatial actions.

In cyberspace yo can find digital places that contain information, people surfing and communicating through the digital networks.

As the physical space, it contains objects (files, images) and structures (laws that rule its behaviour).

(Taken from the web site 'Cybergeography.org

Particularly the study of the spatial nature of the Internet, the World-Wide Web and other electronic 'places' that exist 'behind' our computer screens, popularly referred to as cyberspace.

In this context it refers to the algorithms, conceptual constructions and mathematical background needed to render virtual scenes either 2D or 3D.

Typically they coincide with the columns of a data table. Each column corresponds to one dimension.

For example, if we get information about the name, latitude, longitude, population and occupied surface of a city, we have a problem in 5 dimensions. We can also say that it's 5-dimensional.

The application of techniques of the Geographc Information Systems allows you to integrate data from multiple sources with geographical data. 

This combination of marketing information related with cartographic data allows you to analyse the spatial relationship between data, identify patterns of behaviour, select points of sale, locate customers, measure the results of advertising and promotion and to evaluate the trajectory of a product's life cycle. 

These data can be of any kind: sales figures, revenues, population census, real estate, illness rates, etc. The fundamental issue that distinguishes it from other information systems is that of making the relationship between these data and the geographical coordinates of the Earth surface.

They are mainly used in demography, town planning, natural resources management, business, marketing, logistics and distribution.

In a more informal way we can say that a graph is a set of nodes with links between them called edges or arcs.

In a simple graph there’s only one arc between two nodes. If there’s more than one arc we call it a multigraph. If arcs can be followed in only one specific direction but not in the other we call it a directed graph or digraph and arcs become edges. If arcs begin and end in the same node making a loop, the resulting graph is called a pseudograph. 

Despite a graph seeming a very elementary structure, there are many features of graphs whose study has lead to a complete mathematical theory. (For more information you can take a look at the graph glossary by Chris Caldwell or the introduction to graph theory of the wikipedia). 

There are many ways to represent a graph. There are even complete congresses devoted to discussing how to do it; for example the International Symposium on Graph Drawing

According to Richard Saul Wurman an Information Architect is:

1. the individual who organises the patterns inherent in data, making the complex clear.
2. a person who creates the structure or map of information which allows others to find their personal paths to knowledge.
3. the emerging 21st century professional occupation addressing the needs of the age focused upon clarity, human understanding and the science of the organisation of information.

Information architecture has many things in common with Information Design. For this reason sometimes the two terms are confused.

We consider it as part of Information Visualisation.

(Taken from the book 'Readings in Information Visualization').

It includes any technique , be it visual or not, that facilitates the transaction of information between emitter and receiver.

We consider it as part of Information Visualisation.

Information is a process by which, from the set of accessible data, the subset of relevance  for the subject being informed is extracted and elaborated.

An important meaning of information, simplified yet very useful in Information Visualisation , is the one that defines it as "data within context", i.e. accompanied by metadata that confer it a meaning.

This way one can generate a data table where, for example, each column is associated to a specific metadata that gives it its meaning, for example "date" or "name".

Tha data table is the key element in order to transform data into a graphic representation, since it gathers all the data of the problem with the same semantics into one variable or dimension of the same.

The excess of information that comes with the landslide of available information present nowadays produces occasionally a difficulty in assimilating such a volume in a reduced time.

The consequence is time becoming a very scarce resource and, therefore, getting the attention of the audience becomes a priority in Internet.

Although information is visualized mainly in a visual way, in this context Information Visualisation has to be understood in a more general way; as perception or internalization i.e. understanding.

In principle this includes whatever media could be used to get the understanding, be it graphics, written text, sound, animations, etc.

Information Visualisation relies basically on:

• Human beings receive the bulk of information in a visual way due to the fact that this is the sense with more bandwidth, i.e. the one that yields more quantity of information.
• The symbolic capability of the human brain.

InfoVis includes explicitly the following topics (among others):

• Information Design or Architecture
• Scientific Visualisation
• Graphic representations in general

Pixel is the acronym of "pictorial element" or "picture element".

One pixel is each and every addressable element in a video screen. It's normally controlled by a memory matrix called frame buffer that contains the necessary information to illuminate the pixel with predetermined colour, brightness and luminance.

Usually the palette is a subset of the total number of colours that can be shown on the screen.

With screens able to simultaneously represent millions of colours it's necessary to allocate a series of indices (in many cases 256 -from 0 to 255) to a set of corresponding colours associated to the values that can take the variables they represent.

There's a variety of palettes going form the simplest ones with a few (less than 10) basic colours to the spectral palette that covers all the visible spectrum, passing through graded palettes that have different hues of the same colour.

Selecting the appropriate palette for our purpose is usually crucial for obtaining a good representation of the data

Relevance is a key concept, because it depends on the subject receiving the information.

What is relevant for one person can be useless for another and hence can be disinformation or even 'rubbish'.

Relevance is especially important when speaking about information retrieval.

It comes from the greek words steganós steganos (covered) and graptos graptos  (writing), literally covered writing, in the sense of a hidden thing.

In steganography the text or image in question is invisible although it resides in the interior of an apparently normal piece of other information, like a text an image or a soundtrack.

Unlike cryptography where the message is clearly visible although you need the key to decipher it, in this case the information cannot be seen unless the correct procedure is applied to the text or image where it resides. 

Serendipity has been recently used in connection with Internet, since the large quantity of information available provides chances to find unexpected relevant information while surfing the web.

In Science one speaks about serendipity when the discovery is made by reasons alien to the established research experiments. For example the discovery of penicillin.

The coinage of the term is attributed to the British writer Horace Walpole from a Persian tale "The Three Princes of Serendip" where the heroes make discoveries accidentally.

An example is that of the musical notes, or the letters of text. Signs sometimes are part of a symbol.

The study of signs is called Semiotics.

It's necessary to differentiate between symbol and sign. A symbol implies more than its immediate meaning. Sometimes even the represented concept can have different meanings according to the person considering it.

A flag si aclear example of a symbol. As a sign the flag is just a synonimous of its corresponding state or nation. As a symbol it represents a set of people, institutions, emotions and non rational feelings in a somewhat ambiguous way.

The study of symbols is called symbology while the study of signs is called Semiology.

The rules and principles and elements of a system typically are strongly inter-related. 

Usability is different than utility (ability of something to satisfy a need). The word usability arises in relation with the Human-Computer interface studies.

The interface of a program or of a web site can be useful because it performs the whole range of operations specified, but can be of low usability, for example, due to a high complexity that makes it difficult to be used efficiently for non-trained people.

Visualisation derives, in this context, from the graphic representation of variables associated to the concept that one wants to follow.

For instance, a plot of the fever vs. time allows us to visualize the evolution of the illness. Fever (temperature) and time are the variables. Illness is the concept.

An archetypical case is the desktop metaphor. In it the traditional hierarchical tree of directories and subdirectories is substituted by the graphical interface of folders and files.

A genealogic tree is another typical example of visual metaphor.

In a simplified way we could say that it's data mining adapted to the search of relevant information related to the web.

The scope of web mining covers mainly three topics from knowledge discovery:

• Web Structure Mining (WSM)
  
  
• Web Content Mining (WCM)
  
  
• Web Usage Mining (WUM)

The term was coined by O. Etzioni in 1996

Etzioni, O. (1996). "The World Wide Web: Quagmire or Gold Mine" Communications of the ACM, 39(11), 65-68.