In linguistics, morphology is the study of words, including the principles by which they are formed, and how they relate to one another within a language. Most approaches to morphology investigate the structure of words in terms of morphemes, which are the smallest units in a language with some independent meaning. Morphemes include roots that can exist as words by themselves, but also categories such as affixes that can only appear as part of a larger word. For example, in English the root catch and the suffix -ing are both morphemes; catch may appear as its own word, or it may be combined with -ing to form the new word catching. Morphology also analyzes how words behave as parts of speech, and how they may be inflected to express grammatical categories including number, tense, and aspect. Concepts such as productivity are concerned with how speakers create words in specific contexts, which evolves over the history of a language.

The basic fields of linguistics broadly focus on language structure at different "scales". Morphology is considered to operate at a scale larger than phonology, which investigates the categories of speech sounds that are distinguished within a spoken language, and thus may constitute the difference between a morpheme and another. Conversely, syntax is concerned with the next-largest scale, and studies how words in turn form phrases and sentences. Morphological typology is a distinct field that categorises languages based on the morphological features they exhibit.

The history of ancient Indian morphological analysis dates back to the linguist Pāṇini, who formulated the 3,959 rules of Sanskrit morphology in the text Aṣṭādhyāyī by using a constituency grammar. The Greco-Roman grammatical tradition also engaged in morphological analysis. Studies in Arabic morphology, including the Marāḥ Al-Arwāḥ of Aḥmad b. 'Alī Mas'ūd, date back to at least 1200 CE.

The term "morphology" was introduced into linguistics by August Schleicher in 1859.^{[non-primary source needed]}

The term "word" has no well-defined meaning. Instead, two related terms are used in morphology: lexeme and word-form^{[definition needed]}. Generally, a lexeme is a set of inflected word-forms that is often represented with the citation form in small capitals. For instance, the lexeme eat includes the word-forms eat, eats, eaten, and ate. Eat and eats are therefore different word-forms of the same lexeme. In contrast, eat and eater are distinct lexemes, as they represent two different concepts.

Here are examples from other languages of the failure of a single phonological word to coincide with a single morphological word form. In Latin, one way to express the concept of 'NOUN-PHRASE₁ and NOUN-PHRASE₂' (as in "apples and oranges") is to suffix '-que' to the second noun phrase: "apples oranges-and". An extreme level of the theoretical quandary posed by some phonological words is provided by the Kwak'wala language. In Kwak'wala, as in a great many other languages, meaning relations between nouns, including possession and "semantic case", are formulated by affixes, instead of by independent "words". The three-word English phrase, "with his club", in which 'with' identifies its dependent noun phrase as an instrument and 'his' denotes a possession relation, would consist of two words or even one word in many languages. Unlike most other languages, Kwak'wala semantic affixes phonologically attach not to the lexeme they pertain to semantically but to the preceding lexeme. Consider the following example (in Kwak'wala, sentences begin with what corresponds to an English verb):

kwixʔid-i-da

clubbed-PIVOT-DETERMINER