Any technique to compress speech in order to use less bandwidth when transmitting. Various standardized techniques are used in Europe and the US, most of which employ lossy compression.
GSM 06.10 is used by many European wireless telephones. It uses residual pulse excitation/long-term prediction (RPE/LTP) coding and compresses 160 frames of 13-bit samples into 260 bits. A sample rate of 8 kHz is used. It is being replaced by the Enhanced Full Rate and Adaptive Multi-Rate standards, which provide better speech quality. MELP is the US Department of Defense's mixed excitation linear prediction voice coder; it is also used by NATO.