In linguistics, a word is the smallest element that can be uttered in isolation with objective or practical meaning.
This contrasts deeply with a morpheme, which is the smallest unit of meaning but will not necessarily stand on its own. A word may consist of a single morpheme (for example: oh!, rock, red, quick, run, expect), or several (rocks, redness, quickly, running, unexpected), whereas a morpheme may not be able to stand on its own as a word (in the words just mentioned, these are -s, -ness, -ly, -ing, un-, -ed).
A complex word will typically include a root and one or more affixes (rock-s, red-ness, quick-ly, run-ning, un-expect-ed), or more than one root in a compound (black-board, sand-box). Words can be put together to build larger elements of language, such as phrases (a red rock, put up with), clauses (I threw a rock), and sentences (He threw a rock too, but he missed).