A Pre-Main-Sequence Star (PMS or PMS Object) is a star in an early stage after being a Protostar and before becoming a Main Sequence Star, in other words, before the star is burning Hydrogen in a nuclear reaction. At this early point, the star's energy is from gravitational contraction.
Two distinctions used to classify pre-main-sequence stars and protostars are:
T-Tauri Star (TTS)
Young Stellar Object (YSO)