In music theory, the term period refers to forms of repetition and contrast between adjacent small-scale formal structures such as phrases. In twentieth-century music scholarship, the term is usually used similarly to the definition in the Oxford Companion to Music: "a period consists of two phrases, antecedent and consequent, each of which begins with the same basic motif." Earlier and later usages vary somewhat, but usually refer to notions of symmetry, difference, and an open section followed by a closure. The concept of a musical period originates in comparisons between music structure and rhetoric at least as early as the 16th century.

In Western art music or Classical music, a period is a group of phrases consisting usually of at least one antecedent phrase and one consequent phrase totaling about 8 bars in length (though this varies depending on meter and tempo). Generally, the antecedent ends in a weaker and the consequent in a stronger cadence; often, the antecedent ends in a half cadence while the consequent ends in an authentic cadence. Frequently, the consequent strongly parallels the antecedent, even sharing most of the material save the final bars. In other cases, the consequent may differ greatly (for example, the period in the beginning of the second movement of the Pathetique Sonata).

The 1958 Encyclopédie Fasquelle defines a period as follows:

Another definition is as follows:

And

More recent definitions, especially by American theorists, have tightened the use of the term to restrict the contrast so that the first phrase must end in a half cadence or imperfect authentic cadence and the second a perfect authentic cadence.

A double period is, "a group of at least four phrases...in which the first two phrases form the antecedent and the third and fourth phrases together form the consequent."

When analyzing Classical music, contemporary music theorists usually employ a more specific formal definition, such as the following by William Caplin: