According to Birger Hjørland's definition:
Boundary object is a concept originally introduced by Susan L. Star and James R. Griesemer (1989) to refer to objects that serve an interface between different communities of practice. Boundary objects are an entity shared by several different communities but viewed or used differently by each of them.
Star, S., L. & Griesemer, J. R. (1989). Institutional Ecology, 'Translations' and Boundary Objects: Amateurs and Professionals in Berkeley's Museum of Vertebrate Zoology, 1907-39. Social Studies of Science, 19, 387-420.
Why do I blog this? this is an important concept, and I am often trapped in conversations in which I have to struggle between disciplines and communities. For instance, talking about "locative" with my former linguistics teacher is very different than talking about locative media with LBS designers.
Update: Fabio has a good blogpost about this very concept. He has a good take about it:
I am more convinced than ever that disciplinal purity will unlikely ever be a defining characteristic of our practice, but that this has to be seen under a positive light, as an opportunity to shape theoretical frameworks around fluid, relational models, rather than striving for monolithic, all-encompassing paradigms.