In jazz or popular music, a tune that continues or appears regularly in a piece of music while other parts change or are added. (Cambridge Dictionary)
A short repeated phrase in popular music and jazz, typically used as an introduction or refrain in a song. (Oxford Dictionaries)
A monologue or spoken improvisation, especially a humorous one, on a particular subject. (Oxford Dictionaries)
A rapid energetic often improvised verbal outpouring. (Merriam – Webster)
In jazz and rock music, a riff is a short repeated tune. (Collins Dictionary)
A riff is a short piece of speech or writing that develops a particular theme or idea. (Collins Dictionary)
Riff. Your Dictionary.