A Dictionary of Music and Musicians/Rubato
RUBATO, lit. 'robbed' or 'stolen,' referring to the values of the notes, which are diminished in one place and increased in another. The word is used, chiefly in instrumental music, to indicate a particular kind of licence allowed in order to emphasise the expression. This consists of a slight ad libitum slackening or quickening of the time in any passage, in accordance with the unchangeable rule that in all such passages any bar in which this licence is taken must be of exactly the same length as the other bars in the movement, so that if the first part of the bar be played slowly, the other part must be taken quicker than the ordinary time of the movement to make up for it; and vice versa, if the bar be hurried at the beginning, there must be a rallentando at the end. In a general way this most important and effective means of expression is left entirely to the discretion of the performer, who, it need scarcely be said, should take great care to keep it within due limits, or else the whole feeling of time will be destroyed, and the emphasis so desirable in one or two places will fail of its effect if scattered over the whole composition. Sometimes, however, it is indicated by the composer, as in the 1st Mazurka in Chopin's op. 6 (bar 9), etc. This licence is allowable in the works of all the modern 'romantic' masters, from Weber downwards, with the single exception of Mendelssohn, who had the greatest dislike to any modification of the time that he had not specially marked. In the case of the older masters, it is entirely and unconditionally inadmissible, and it may be doubted whether it should be introduced in Beethoven, although many great interpreters of his music do not hesitate to use it. [See Tempo.]
[ J. A. F. M. ]