Source : Webster's Revised Unabridged Dictionary (1913)

Rubato \Ru*ba"to\, a. [It.]
   Robbed; borrowed.

   {Temple rubato}. [It.] (Mus.) Borrowed time; -- a term
      applied to a style of performance in which some tones are
      held longer than their legitimate time, while others are
      proportionally curtailed.

Source : WordNet®

rubato
     n : a flexible tempo; not strictly on the beat