Galaxy

A galaxy is a collection of stars and planets following a circular orbit around a central point, a dense cluster of millions of stars, nebulae and various other matter which is held in place by a massive gravitational attraction (such as a super massive blackhole). Galaxies can come in groups, including the Local Group. According to Colonel Samantha Carter, there are hundreds of millions of galaxies in the known Universe. (SG1: "Reckoning, Part 2")

However this is a gross underestimation as there are actually approximately between 170 and 200 billion galaxies in the known universe, with newer estimates being 2 trillion.

Galaxies can take on many shapes and sizes, our own galaxy the Milky Way is a Spiral Galaxy so is naturally divided into a number of spiral arms, never the less it is a common practice as means of general galactic charting by many races to divided the Milky Way into four equal Quadrants.

In addition to the main body of a galaxy, many galaxies also have a number of smaller satellite galaxies and globular star clusters which seemingly orbit the main galactic body. (ST reference: Star Charts)

Known galaxies[]

Andromeda Galaxy
Kasteroborous
Triangulum Galaxy
unnamed galaxies
Alteran Home Galaxy
Ida Galaxy
Kaliam Galaxy
Milky Way Galaxy
Othalla Galaxy
Pegasus Galaxy
Koyila Galaxy
Vennix Galaxy
NGC 3109 Galaxy
An Unnamed Galaxy four million light years away, where SG-1 appeared after the Battle of the Vorash system. This galaxy was known to the Replicators, so may also have been known to the Asgard as well. It may even have been Ida.
The Ancients launched a number of unmanned ships on an intergalactic mission to seed Stargates throughout the universe. Although it is unclear exactly how many galaxies have been seeded by these ships, a star map on Destiny reveals that a minimum of 38 galaxies were seeded after Pegasus. It is likely that many additional galaxies have been seeded because many Stargate seeding ships were dispatched in the first place. No one knows how many ships were dispatched to seed Stargates throughout the universe, and it is unclear how many galaxies have been visited by Destiny. (SGU: "Air, Part 1")
Here are the list of known galaxies traveled by Destiny: