amplifier

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that amplifies, enlarges, or extends.
• noun An electronic device that is used to increase the magnitude of an electrical signal.
• noun A device that is used to increase the magnitude of an information-carrying signal.

from The Century Dictionary.

• noun One who amplifies or enlarges.
• noun A lens placed in the tube of a microscope between the object-glass and the eyepiece. See microscope.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who or that which amplifies.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Anything that amplifies, or makes something larger or more intense.
• noun electronics An appliance or circuit that increases the strength of a weak electrical signal without changing the other characteristics of the signal.
• noun US, music An amp, specif. a particular type of speaker used to amplify voices and musical instruments at live performances.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun electronic equipment that increases strength of signals passing through it