"Definitions" of Cognitive Science:
The study of the nature of various mental tasks and the processes that enable them to be performed.
-- The American Heritage Dictionary of the English Language, Fourth Edition. 2000
The field of science concerned with cognition; includes parts of cognitive psychology and linguistics and computer science and cognitive neuroscience and philosophy of mind.
-- WordNet 2.0
The scientific study either of mind or of intelligence. Practically every introduction to cognitive science also stresses that it is highly interdisciplinary; it is often said to consist of, take part in, and collaborate with psychology (especially cognitive psychology), linguistics, neuroscience, artificial intelligence (neural network research in particular), robotics, anthropology, biology (including biomechanics) and philosophy (especially philosophy of mind and philosophy of mathematics, but also with applications in philosophy of science).
The interdisciplinary study of mind and intelligence, embracing philosophy, psychology, artificial intelligence, neuroscience, linguistics, and anthropology. Its intellectual origins are in the mid-1950s when researchers in several fields began to develop theories of mind based on complex representations and computational procedures. Its organizational origins are in the mid-1970s when the Cognitive Science Society was formed and the journal Cognitive Science began. Since then, more than sixty universities in North America and Europe have established cognitive science programs and many others have instituted courses in cognitive science.
-- Stanford Encyclopedia of Philosophy
My words. In short, it is "(scientific) thinkings about thinkings". Hmm...
See Google's Web Definitions for Cognitive Science.