Tempo Rubato - Types of Rubato

Types of Rubato

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 was 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 and Rubato).

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).
—Music notation and terminology (1921) by Karl Wilson Gehrkens A tempo rubato. Lit. "in robbed time", i. e. time in which, while every bar is of its proper time value, one portion of it may be played faster or slower at the expense of the remaining portion, so that, if the first half be somewhat slackened, the second half is somewhat quickened, and vice versa. With indifferent performers, this indication is too often confounded with some expression signifying ad libitum.
—A dictionary of foreign musical terms and handbook of orchestral instruments (1907) by Tom S. Wotton

The opinion given by Tom S. Wotton, that "every bar has its proper time value" may be regarded as an inaccurate description: Karl Wilson Gehrkens mentions "duration taken from one measure and given to another" which implies bars of differing duration. Rubato relates to phrasing; and since phrases often go over multiple bars; it is often impossible (and also not desired) for each bar to be identically long.