a short concerto with a solo instrument
Literally, the little concerto. This refers to the group of soloists in a Baroque concerto or concerto grosso. Even in a solo concerto, there is still a "group" in the concertino, because of the presence of the continuo.
A short concerto; the small group of solo instruments used in a concerto grosso
Solo group of instruments in the Baroque concerto grosso.
A concertino is the smaller group of instruments in a concerto grosso. This is opposed to the ripieno which is the larger group contrasting with the concertino.
A concertino (or Konzertstück) is a short concerto. It normally takes the form of a one-movement musical composition for solo instrument and orchestra, though some concertinos are written in several movements played without a pause.