The 1958 Encyclopédie Fasquelle defines a period as follows:

• "A complex phrase, in which the various parts are enchained."

Another definition:

• "In traditional music...a group of measures comprising a natural division of the melody; usually regarded as comprising two or more contrasting or complementary phrases and ending with a cadence." (Harvard Dictionary of Music, 1969)

Source

• Nattiez, Jean-Jacques (1990). Music and Discourse: Toward a Semiology of Music (Musicologie générale et sémiologue, 1987). Translated by Carolyn Abbate (1990). ISBN 0691027145.
  • (1969). Harvard Dictionary of Music.