A bar is a feature of some spiral galaxies which are said to be barred, i.e., barred spiral galaxies (or unbarred if not, i.e., unbarred spiral galaxy). Spiral galaxies tend to have a region with a lot of stars in the center (a bulge), which can be roughly spherical, or a flatter (disk or lump) shape, or can have a longer dimension, forming a straight arm joining the inner ends of the spiral arms, in which case it is known as a bar. The Milky Way is now presumed to have a bar. The abbreviation SB in the galaxy classification is used for barred spiral galaxies as opposed to S without the "B" for other spiral galaxies. The dynamics that produce this morphology are of interest. Bars are thought to contain star-forming regions, which is reasonable if the visible bar-appearance is from the bright light of many early type stars in the region.
Lenticular galaxies can also show bars, i.e., barred lenticular galaxy versus unbarred lenticular galaxy. Also, irregular galaxies, i.e., a barred irregular galaxy.
The term bar is also used in a totally unrelated manner, for a unit of pressure, intended to be close to the pressure of Earth atmosphere.