Interdisciplinarity is when two or more subjects (academic disciplines) join up. This happens when a problem overlaps traditional academic boundaries. Other terms which mean almost the same thing are multidisciplinarity and crossdisciplinarity.
An examples makes clear what is meant:
Other examples include artificial intelligence, cultural studies, cybernetics, computational linguistics, biomedical engineering, and so on. Physical chemistry, biochemistry and astrophysics must have been some of the first.