The term dwarf galaxy is used for galaxies much smaller
than the Milky Way, i.e.,
of a few billion stars at most and as small as 100 parsecs
across. They can be any of the normal galaxy types
(dwarf elliptical, dwarf irregular, dwarf spiral)
or a dwarf spheroidal galaxy, more sphere-shaped,
like a typical globular cluster but more extensive
(and perhaps containing multiple globular clusters),
and dwarf galaxy classifications have been defined.
Many known dwarf galaxies are satellite galaxies.
Additional specific terms:
blue compact dwarf galaxy (BCD or blue compact galaxy or BCG) - with young hot stars.
ultra-compact dwarf galaxy (UCD) - a width in the 100-parsec range.
hobbit galaxy - small, faint dwarf galaxy.
(The term compact galaxy (CG) came into use meaning it has
a high surface brightness compared to its apparent diameter,
which can be the result of a very high density of stars.)