A Level English Language Website
What is a word?
The word is the smallest grammatical unit which is capable of occurring on its own as an utterance. In speech it can be preceded and followed by a pause; in writing it can be preceded and followed by a space.
That is a very bare answer. I suggest you follow it up by reading one or more of the following sources:
Crystal, David The Cambridge Encyclopedia of Language. Cambridge: Cambridge University Press 1987. p. 91.
Katamba, F. English Words. London: Routledge 1994. Chapter 2.
McArthur, T. The Oxford Companion to the English Language. (Abridged edn.) Oxford: Oxford University Press. 1996 pp.1024-33.