A very faint galaxy. Dwarf galaxies are either small or just not very bright, or sometimes both.
Small, low luminosity galaxies that are associated with larger spiral galaxies and may make up part of a galactic halo.
A small galaxy usually containing any number of stars between a million and several billion. There is no official size below which a galaxy is designated a dwarf but any galaxy with a diameter below 30 000 light years can be considered a dwarf.