Most italian musical indications and concepts are easy to translate in one or two words (largo, presto, crescendo, ...) but it is difficult to find even an approximation to rubato.
"Flexible time" is probably the closest translation that gets the point across. However, "stolen time" or "robbed time", as Raskolnikov suggests, would be the most accurate translation. The English speaker unfamiliar with it would just need to think about it a little: stealing time from some notes and giving it to others. Heck, maybe "Robin Hood time" would be a good name too :P
How about some Elasticity?
Possible Usages: In Elastic Tempo; Elastic; With Elasticity
A couple of subjective suggestions, none of which I find very good:
The problem is that this freedom must be within keeping the average tempo.
I think "flexible tempo" could work.
According to Wikipedia, this is
robbed as in
A musician playing with rubato would "rob" from a part of the phrase they would want to be slower, and speed up later on according to the music. For example if you had a rising passage, you may wish to slow down to emphasize it, but resume your normal tempo after.
And this is further outlined by Anthony Tomassini, Chief Classical Music Critic for New York Times:
Let me start my answer by trying to define, for those who need some help, this loose term "rubato." It describes the practice of playing with expressive and rhythmic freedom. Specifically "tempo rubato" means "robbed time," a term that indicates that strict time in a passage is ignored temporarily while some time is "robbed" from one passage or group of notes and given to another. There are intense debates within the field over the degree of rubato that is considered tasteful and appropriate.
Google Translate (which doubles as a dictionary) says it means stolen but can also mean bent.