synthesizer

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

• noun One that synthesizes.
• noun Music An electronic instrument, often played with a keyboard, that combines simple waveforms to produce more complex sounds, such as those of various other instruments.

from The Century Dictionary.

• noun In acoustics, an instrument for the production of complex tones of predetermined composition.

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

• noun music An electronic instrument that creates its sounds with electronics and has a keyboard.
• noun music An electronic instrument module that creates its sounds with electronics and does not have any keyboard.
• noun electronics An electronic circuit that generates an electronic signal oscillation with accurate timing from a reference oscillator.
• noun electronics An electronic device that generates electronic signal patterns to test an electronic circuit.

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

• noun (music) an electronic instrument (usually played with a keyboard) that generates and modifies sounds electronically and can imitate a variety of other musical instruments
• noun an intellectual who synthesizes or uses synthetic methods