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).
In the Minimalist school of theoretical syntax, words (also called lexical items in the literature) are construed as "bundles" of linguistic features that are united into a structure with form and meaning. For example, the word "koalas" has semantic features (it denotes real-world objects, koalas), category features (it is a noun), number features (it is plural and must agree with verbs, pronouns, and demonstratives in its domain), phonological features (it is pronounced a certain way), etc.
In languages with a literary tradition, there is interrelation between orthography and the question of what is considered a single word. Word separators (typically spaces) are common in modern orthography of languages using alphabetic scripts, but these are (excepting isolated precedents) a relatively modern development (see also history of writing).
In English orthography, compound expressions may contain spaces. For example, ice cream, air raid shelter and get up each are generally considered to consist of more than one word (as each of the components are free forms, with the possible exception of get).
Not all languages delimit words expressly. Mandarin Chinese is a very analytic language (with few inflectional affixes), making it unnecessary to delimit words orthographically. However, there are many multiple-morpheme compounds in Mandarin, as well as a variety of bound morphemes that make it difficult to clearly determine what constitutes a word.