在语言学中，单词（word，又称为词、词语）是能独立运用并含有语义内容或语用内容（即具有表面含义或实际含义）的最小单位。单词的集合称为词汇、术语，例如：所有中文单词统称为「中文词汇」，医学上专用的词统称为「医学术语」等。词典是为词语提供音韵、词义解释、例句、用法等等的工具书，有的词典只修录特殊领域的词汇。

从语法单位来说，单词比语素大（语素含有语意，但不一定能独立运用），而又比词组小（又称短语，由单词构成）。一个单词可能只会有一个语素，（例如英语的 oh!、rock、、red、quick、run、expect 等），但也有可能有很多个（例如 rocks、redness、quickly、running、unexpected）。而语素可能不能独立存在（例如 -s，-ness，-ly，-ing，un-，-ed）。一个复杂的单词通常会包括一个词根和一或多个词缀（rock-s，red-ness，quick-ly，run-ning，un-expect-ed），但合成词可能会有多于一个词根（black-board（黑板），rat-race（激烈的竞争））。构词学是研究词的内部结构和形成方式等的语言学分支。

In linguistics, a word is the smallest element that may be uttered in isolation with semantic or pragmatic content (with literal 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, rat-race). Words can be put together to build larger elements of language, such as phrases (a red rock), clauses (I threw a rock), and sentences (He threw a rock too, but he missed).