Rubato

Tempo rubato [ˈtɛmpo ruˈbaːto] (free in the presentation, Italian for: stolen time) 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.

[...] this loose term "rubato." It describes the practice of playing with expressive and rhythmic freedom. Specifically "tempo rubato" [...] some time is "robbed" from one passage or group of notes and given to another.

One can distinguish two types of rubato: in one the tempo of the melody is flexible, while the accompaniment is kept in typical regular pulse (yet not rigidly in mechanical fashion; but adjusting to the melody as necessary—see below). Another type affects melody and accompaniment. While it is often associated with music of the Romantic Period, classical performers frequently use rubato for emotional expressiveness in all kinds of works.

Rubato, even when not notated, is often used liberally by musicians; e.g. singers frequently use it intuitively to let the tempo of the melody expressively shift slightly and freely above that of the accompaniment. This intuitive shifting leads to rubato's main effect: to make music sound expressive and natural. Frédéric Chopin is often mentioned in context with rubato (see Chopin's technique and performance style).

While rubato is often loosely taken to mean playing with expressive and rhythmic freedom; it was traditionally used specifically in the context of expression by speeding up and then slowing down. In the past expressive and free playing (beyond only rubato) was often associated with the terms "ad libitum":

Tempo rubato (or a tempo rubato) means literally in robbed time, i.e., duration taken from one measure or beat and given to another, but in modern practice the term is quite generally applied to any irregularity of rhythm or tempo not definitely indicated in the score.
The terms ad libitum, (ad lib.), a piacere, and a capriccio, also indicate a modification of the tempo at the will of the performer. Ad libitum means at liberty; a piacere, at pleasure; and a capriccio, at the caprice (of the performer).

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Wikipedia