(1) A power amplifier which increases the amplitude of a voltage,current, or power signal,
(2) any direct radiator speaker, or
(3) the term used to describe the chassis loudspeaker, mid-range unit, or tweeter elements of a loudspeaker system (as opposed to speaker system which covers both cabinet and drivers.)
(4) A software program which enables communication between a particular make and model of a hardware device and the computer’s operating system, usually necessary for some kind of I/O device such as a soundcard, printer, or scanner. The problem of outdated hard-disks and soundcard drivers is particularly problematic.