The **mean opinion score (MOS)** is a [[subjective test]] for the evaluation of [[what is speech synthesis|speech synthesis]] systems where a group of human listeners is asked to rate the quality of synthesized speech produced by a TTS system on a scale from 1 to 5. The ratings are then averaged across all listeners to obtain the final MOS. MOS is a widely used metric in the speech synthesis community and is often used to compare the performance of different TTS systems or different configurations of the same system. However, it should be noted that MOS scores can be influenced by various factors such as speaker variability, text complexity, and listener fatigue, among others. Therefore, it is important to carefully design and conduct subjective tests to obtain reliable and meaningful MOS scores.