A fleet is the largest formation of vessels in a navy, subordinate to no other military units except its overall naval command.
While the term 'fleet' is referring to a specific defined component of a navy, in many contexts the colloquial usage of the word is often referring to every vessel within a navy, i.e. "she's the fastest vessel in Martian Congressional Republic Navy", "United Nations Navy has an aging fleet", "the Outer Planets Alliance wants to develop its fleet" etc. making it virtually synonymous with 'navy'.
A fleet is usually a permanent formation, although examples of temporary and even ad hoc fleets exist. Dating back to their nautical origins, permanent formations were often titled by their areas of geographic responsibility such as "Atlantic fleet", "Mediterranean fleet", et al. or by more arbitrary terms such as "1st Fleet", "2nd Fleet", etc.
Notable examples include:
- Earth-Mars Coalition Navy - While referred to as a 'navy', it was a permanent multi-governmental fleet with forces contributed by the Martian Congressional Republic Navy and the United Nations Navy in for the express purpose of joint-policing The Sol System.
- Augusto Nguyen - A fleet of approximately a dozen UNN vessels under the command of Augusto Nguyen, with the Agatha King serving as his flagship, over the Moon Io.
- Souther - A separate UNN fleet that would also be brought to Io. Admiral Souther used the Okimbo as his flagship.
- Admiral Duarte - A fleet of MCRN vessels under the command of Admiral Duarte, last referenced passing through the Slow zone. The flagship of which is presumably the Barkeith.
- Slow Zone Incident - An ad hoc fleet that formed as the result of the the UN, the MCR, and the OPA all pursuing Jim Holden through The Ring, along with several other motivations. The Behemoth will eventually become the flagship, almost by default.
There is no predefined composition of a fleet, consisting only of whatever available vessels that are required to fulfill its objectives and operations.
Within every fleet though, one vessel will designated as the flagship, from which the commanding officer (typically a full admiral) of the fleet will conduct command and control. While the flagship is often the largest, most heavily-armed, or otherwise most significant ship of the fleet, but it ultimately a temporary designation that can be transferred to any vessel the commanding officer may need to command the fleet from.