public-address system
a system of one or more microphones, amplifiers, and loudspeakers for increasing the sound level of speech or music, used in auditoriums, public gatherings, etc.
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
sound-amplification system
A combination of one or more microphones, amplifiers, loudspeakers, and associated electronic controls; used to increase the level of a sound source so that it may be heard clearly in all parts of an auditorium, large room, open-air theater, etc.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.