Tempo rubato (Italian for 'stolen time'; UK: , US: , Italian: [ˈtɛmpo ruˈbaːto];) is a musical term referring to expressive and rhythmic freedom by a slight speeding up and then slowing down of the tempo of a piece at the discretion of the soloist or the conductor. Rubato is an expressive shaping of music that is a part of phrasing. While rubato is often loosely taken to mean playing with expressive and rhythmic freedom, it was traditionally used specifically in the context of expression as speeding up and then slowing down the tempo.
https://en.wikipedia.org/wiki/Tempo_rubato