The Collaborative International Dictionary of English v.0.48:

Rubato \Ru*ba"to\, a. [It.]
   Robbed; borrowed.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rubato
    n 1: a flexible tempo; not strictly on the beat