in geology, the basic unit of the geologic time scale; during these spans of time specific systems of rocks were formed. Originally, the sequential nature of defining periods was a relative one, originating from the superposition of corresponding stratigraphic sequences and the evidence derived from paleontological studies. With the advent of radiometric dating methods, absolute ages for various periods can be determined.

in music, a unit of melodic organization made up of two balanced phrases in succession; the first phrase, called the antecedent, comes to a point of partial completeness; it is balanced by the consequent, a phrase of the same length that concludes with a sense of greater completeness. The phrase length varies but is typically 2, 4, or 8 measures in moderate tempo; it can be 16 measures in very fast tempos. A double period consists of two periods, with a stronger cadence at the end of the second period; this four-phrase unit often constitutes an entire section.

Periodic structure is most apparent in music that exhibits regular phrases, particularly in homophonic song forms and dances. Larger forms with more extensive musical development, such as the sonata and the rondo, often have a structure that is at least partially periodic; periodic structure is less likely to be found in polyphonic (polyphony) textures such as the fugue.