The term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:

Galaxy formation and evolution

Galactic spheroid

Galactic bulge

the dark matter halo

Galactic coordinate system

Galactic corona

Disc galaxy

the galactic spheroid (stars)

the galactic corona (hot gas, i.e. a plasma)

The distinction between the halo and the main body of the galaxy is clearest in spiral galaxies, where the spherical shape contrasts with the flat disc. In an elliptical galaxy, there is no sharp transition between the body of the galaxy and the halo.