In popular music, chorus is used to mean the refrain of a song, which often sharply constrasts the verse melodically, rhythmically, and harmonically, and assumes a higher level of dynamics and activity, often with added instrumentation. Chorus form, or strophic form, is a sectional and/or additive way of structuring a piece of music based on the repetition of one formal section or block played repeatedly.
When two or more sections of the song have basically identical music and lyrics these sections are probably instances of the chorus or similar at least. (Courtesy of Wikipedia)
The chorus is the repeated section of a song that usually contains the title.
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