A useful definition:

 

From The Association for Computational Linguistics

http://www.aclweb.org/archive/what.html

 

                           What is Computational Linguistics?

 

Simply put, computational linguistics is the scientific study of language from a computational perspective.  Computational linguists are interested in providing computational models of various kinds of linguistic phenomena.  These models may be "knowledge-based" ("hand-crafted") or "data-driven" ("statistical" or "empirical").  Work in computational linguistics is in some cases motivated from a scientific perspective in that one is trying to provide a computational explanation for a particular linguistic or psycholinguistic phenomenon; and in other cases the motivation may be more purely technological in that one wants to provide a working component of a speech or natural language system.  Indeed, the work of computational linguists is incorporated into many working systems today, including speech recognition systems, text-to-speech synthesizers, automated voice response systems, web search engines, text editors, language instruction materials, to name just a few.