Tempo rubato

Tempo rubato (Italian "robbed time") is a musical term for slightly speeding up or slowing down the tempo as well as altering the relationships among the written note values. The amount of change in tempo is decided by the conductor or a solo musician. Rubato, even when not denoted, is often used liberally by many singers for added musical affect by singing at a slightly different tempo than their accompaniment.

Chopin used rubato to make the left hand(the accompaniment) fit around the right hand (the main melody) as opposed to the left hand driving the peice and thus providing a steady rhythm.