From The RadioReference Wiki
"Vocoder" is a contraction of "voice" and "encoder."
Abbreviation for voice-coder. A device that usually consists of a speech analyzer, which converts analog speech waveforms into narrowband digital signals, and a speech synthesizer, which converts the digital signals into artificial speech sounds. For COMSEC purposes, a vocoder may be used in conjunction with a key generator and a modulator-demodulator to transmit digitally encrypted speech signals over narrowband voice communications channels. These devices are used to reduce the bandwidth requirements for transmitting digitized speech signals. Some analog vocoders move incoming signals from one portion of the spectrum to another portion.