A galaxy can be one of two related things:
A kind of Azimuth Urbit identity that sits at the top of the identity hierarchy. Galaxies, in this sense, have the power to issue stars, meaning that all other kinds of identities ultimate derive from galaxies. Galaxies also form a Senate, the governing body of Azimuth. The Senate has the power to update the logic of Azimuth by majority vote.
A ship on the Arvo network whose identity is a galaxy in the former definition. In this sense, galaxies act as infrastructure for the Arvo network, providing Ames-related services, such as peer-to-peer routing and distributing over-the-air software updates. Stars rely on galaxies for these services in the way that planets rely on stars.
Galaxies are 8-bit, representable by numbers 0 to 255. This means that there are 256 possible galaxies; each of these galaxies is able to issue 255 stars. Galaxies have one-syllable names like ~mul or ~lux.