A word is a basic element of language that carries meaning, can be used on its own, and is uninterruptible. Despite the fact that language speakers often have an intuitive grasp of what a word is, there is no consensus among linguists on its definition and numerous attempts to find specific criteria of the concept remain controversial. Different standards have been proposed, depending on the theoretical background and descriptive context; these do not converge on a single definition. Some specific definitions of the term "word" are employed to convey its different meanings at different levels of description, for example based on phonological, grammatical or orthographic basis. Others suggest that the concept is simply a convention used in everyday situations.

The concept of "word" is distinguished from that of a morpheme, which is the smallest unit of language that has a meaning, even if it cannot stand on its own. Words are made out of at least one morpheme. Morphemes can also be joined to create other words in a process of morphological derivation. In English and many other languages, the morphemes that make up a word generally include at least one root (such as "rock", "god", "type", "writ", "can", "not") and possibly some affixes ("-s", "un-", "-ly", "-ness"). Words with more than one root ("[type][writ]er", "[cow][boy]s", "[tele][graph]ically") are called compound words. Contractions ("can't", "would've") are words formed from multiple words made into one. In turn, words are combined to form other elements of language, such as phrases ("a red rock", "put up with"), clauses ("I threw a rock"), and sentences ("I threw a rock, but missed").

In many languages, the notion of what constitutes a "word" may be learned as part of learning the writing system. This is the case for the English language, and for most languages that are written with alphabets derived from the ancient Latin or Greek alphabets. In English orthography, the letter sequences "rock", "god", "write", "with", "the", and "not" are considered to be single-morpheme words, whereas "rocks", "ungodliness", "typewriter", and "cannot" are words composed of two or more morphemes ("rock"+"s", "un"+"god"+"li"+"ness", "type"+"writ"+"er", and "can"+"not").

Since the beginning of the study of linguistics, numerous attempts at defining what a word is have been made, with many different criteria. However, no satisfying definition has yet been found to apply to all languages and at all levels of linguistic analysis. It is, however, possible to find consistent definitions of "word" at different levels of description. These include definitions on the phonetic and phonological level, that it is the smallest segment of sound that can be theoretically isolated by word accent and boundary markers; on the orthographic level as a segment indicated by blank spaces in writing or print; on the basis of morphology as the basic element of grammatical paradigms like inflection, different from word-forms; within semantics as the smallest and relatively independent carrier of meaning in a lexicon; and syntactically, as the smallest permutable and substitutable unit of a sentence.

In some languages, these different types of words coincide and one can analyze, for example, a "phonological word" as essentially the same as "grammatical word". However, in other languages they may correspond to elements of different size. Because of this unclear status, some linguists propose avoiding the term "word" altogether, instead focusing on better defined terms such as morphemes.

Dictionaries categorize a language's lexicon into individually listed forms called lemmas. These can be taken as an indication of what constitutes a "word" in the opinion of the writers of that language. This written form of a word constitutes a lexeme. The most appropriate means of measuring the length of a word is by counting its syllables or morphemes. When a word has multiple definitions or multiple senses, it may result in confusion in a debate or discussion.

One distinguishable meaning of the term "word" can be defined on phonological grounds. It is a unit larger or equal to a syllable, which can be distinguished based on segmental or prosodic features, or through its interactions with phonological rules. In Walmatjari, an Australian language, roots or suffixes may have only one syllable but a phonologic word must have at least two syllables. A disyllabic verb root may take a zero suffix, e.g. luwa-ø 'hit!', but a monosyllabic root must take a suffix, e.g. ya-nta 'go!', thus conforming to a segmental pattern of Walmatjari words. In the Pitjantjatjara dialect of the Wati language, another language from Australia, a word-medial syllable can end with a consonant but a word-final syllable must end with a vowel.

In most languages, stress may serve a criterion for a phonological word. In languages with a fixed stress, it is possible to ascertain word boundaries from its location.