A pre-main-sequence star (PMS or pre-MS or PMS object)
is a star in an early stage, specifically, before it has become a
main sequence star (before its hydrogen burning). At this early
point, the star's energy is from gravitational contraction
(Kelvin-Helmholtz mechanism).
Usage of terms vary but the term PMS generally indicates a phase that
is distinct from and subsequent to the phase termed protostar.
The PMS phase is after the bulk of the accretion that gave the
star its mass, and after the star was directly embedded in the
nebula that formed it.
The phase includes the Hayashi track and possibly the Henyey track,
and the stars develop accretion disks.
For early stars, the PMS phase can be so short that it
is hardly considered a phase, i.e., hydrogen burning develops
very rapidly.
Some PMS types: