(small, cool main-sequence star in stellar class M)
An M dwarf is main-sequence M-type star.
The term red dwarf definitely includes M dwarfs,
but sometime that term is also meant to include some or all
main-sequence K-type stars (K dwarfs).
M dwarfs are very common,
comprising about three quarters of all main sequence stars,
and it is estimated that about a quarter of them host a planet
in their habitable zone.
Despite being so plentiful, they are so dim that none can be seen
with a telescope, including the closest star to the Sun,
which is a red dwarf.
Early M dwarfs
(the hotter ones) are now considered
likely places to observe extra-solar planets within a
habitable zone.
But given such stars' slow evolution,
such planets are more likely to
have experienced a period of high
stellar activity (such as severe coronal mass ejections)
long enough to remove the planet's atmosphere,
less likely to leave the planet habitable.
M dwarf examples include: