mid-14c., "country, territory, region, political or administrative division of a country," from Old French province "province, part of a country; administrative region for friars" (13c.) and directly from Latin provincia "territory outside Italy under Roman domination," also "a public office; public duty," a word of uncertain origin. It commonly is explained as pro- "before" + vincere "to conquer" (from nasalized form of PIE root *weik- (3) "to fight, conquer"); but this does not suit the earliest Latin usages. Compare Provence. Meaning "one's particular business or expertise" is from 1620s.
Originally, a country of considerable extent which, being reduced under Roman dominion, was remodeled, subjected to the rule of a governor sent from Rome, and charged with such taxes and contributions as the Romans saw fit to impose. The earliest Roman province was Sicily. [Century Dictionary]