The fact is, there are at least two key definitions, usually referred to as "little IA" and "Big IA". Little IA focuses on the design of "findability" -- the taxonomy, classification, and search of an information pool. Big IA embraces a larger sphere -- including navigation, sometimes page layout and functionality, and other aspects of the information access. Interaction designers claim "big IA" is just user interaction design. Others accuse big IA of being "the design of everything" and therefore nothing at all.
I confess to being a big IA. My focus is larger than just findability and often covers a wide variety of topics that cross the boundaries to other professions (such as interaction design, database design, solution architecture, etc). What makes it "IA" is the core focus on the information and how it is used to serve and influence the audience.
So when people ask me what I do and what that means, I say I am an information architect and that information architecture is, quite simply, the structural design of an information space. This definition consists of two components: structural design and information spaces.
- Information architecture, like real architecture, focuses on the high-level design of the structure, understanding its purpose, its use, its layout, and its evolution. But instead of buildings we design information spaces. Information architects are no more web designers than architects are interior decorators. That is not to say an web designer can't be an IA or an interior decorator can't be an architect. But the roles are different and operate on a different scale.
- The thing being designed is an information space. By this we mean all of the content related to and used to achieve the business goal. People tend to think of the information space in terms of recognized boundaries: web sites*, catalogs, a book... However, the physical representation is only one aspect of the design, not a bounding factor. So for a physical product, the information space covers a broad spectrum of media, including user's manuals, packaging, online support, advertising, and even word-of-mouth that can affect people's opinion of the product. In practice, an IA usually has control over only a limited portion of the total space. But it is useful to consider the larger global space to understand what alternatives are available and what falls outside the boundary of controlled content that might influence the results. Going back to the physical architecture analogy, does an architect design a building, or the building and the environment (landscaping, gardens, roadways) that it exists within? At its best, I believe architecture must consider both the physical structure and the environment it operates in.
* Footnote: Some claim that IA only applies to the web, but there is no reason to believe the processes for designing the structure of web information differs significantly from the design of print or other media, although the resulting structures are different.