Acronym for group ID. In a Unix-type operating system, the GID is a number which identifies a user as a member of one or more groups. A group is a collection of user ID's (UID) that may share access to certain files or resources. This is useful for allowing collaboration, or in restricting certain resources to a subset of the entire user base. A user may have more than one group ID. Group level access is one of the access types set using the chmod command. The effective group ID of a running process can be changed by a call to the setgid function- similar to the use of the setuid command.