A Legatus Legionis was the commander of an individual Roman Legion. They were often of senatorial rank, and would be granted the posting by a Consul, Praetor, or provincial governor, so as to deputise for them. Legatus, derived from the Latin word legare, meaning "to represent", would represent the authority of the Senate in the legion. By Imperial times, the post of Legatus Propraetore was created by Augustus to stand-in as his deputy in an imperial-run province, while Proconsuls and Propraetors ran senatorial provinces at the beginning of the Principate or Imperial era of Roman history.
A Legatus Iuridicus (judicial legate) was a trained lawyer who would act as an advisor for the provincial governor in matters of civil law.
Legati was also the term for Roman diplomats to foreign nations, as they represented the Senate and People of Rome abroad.