ETYM Italian, p. pr. of diminuere to diminish.
(Music) In a gradually diminishing manner; with abatement of tone; decrescendo; -- expressed on a music score by 'Dim.,' or 'Dimin.'.

In musical notation, indicating a decrease in the loudness of a passage by constant degrees. Diminuendo poco a poco means that the diminuendo should progress more gradually. An alternative marking is “decrescendo”. The opposite marking is “crescendo”.