They generally designate a gradual change in tempo; for immediate tempo shifts, composers normally just provide the designation for the new tempo. (Note, however, that when Più Mosso or Meno Mosso appears in large type above the staff, it functions as a new tempo, and thus implies an immediate change.)
Origin
Italian.
Definition of meno in US English:
meno
adverbˈmenō
Music
(in directions) less.
〔乐〕（演奏演唱指示）较小地，较少地
Example sentencesExamples
They generally designate a gradual change in tempo; for immediate tempo shifts, composers normally just provide the designation for the new tempo. (Note, however, that when Più Mosso or Meno Mosso appears in large type above the staff, it functions as a new tempo, and thus implies an immediate change.)