The Booster rocket reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)
A Booster in space-related applications is usually a solid fuelrocket-shaped device that is attached to the actual rocket to provide additional boost in the initial phase of the rocket's flight.
There are also liquid fuel boosters (see also Ariane 4), these are called liquid boosters.