Unit may be used as a generic term for any military organization; in proper usage, however, it refers to an organization smaller than a formation, generally divided into sub-units. An Infantry Battalion is an example of a unit, being composed of companies, and grouped into brigades or higher formations.

Typical Units

The following are "typical" units of the various corps/services/branches:

Infantry: battalion - a grouping of companies.
Armoured: regiment - a grouping of squadrons.
Cavalry: regiment - a grouping of squadrons.
Artillery: regiment (brigade was also common before 1940) - a grouping of batteries.
Signals: regiment - a grouping of squadrons.