Tempo rubato (Italian stolen time) is a musical term for slightly speeding up or slowing down the tempo of a piece at the discretion of the soloist or the conductor. It also requires the use of altering the relationships among the written note values and the played ones. While it is often associated with music of the Romantic Period, good classical performers frequently use it for emotional expressiveness (in all kinds of works) and to enhance/idealize the rhythm (softening edges), while remaining stylistically viable and in good taste.
Rubato, even when not notated, is often used liberally by musicians; e.g. singers frequently use it to let the tempo of the melody expressively shift slightly and freely (to and fro) above than that of the accompaniment. The shifting is used to reach the aim of rubato: to make music sound expressive and natural. Frédéric Chopin is often mentioned in context with rubato (see Chopin and Rubato).
