From Wikipedia.

The Imperius Curse places the subject in a trance or dream-like state, and leaves him or her subject to total control by the caster. The experience of being controlled by this curse is described as a complete, wonderful release from any sense of responsibility or worry over one's actions, at the price of one's free will. Resisting the effect of the curse is possible, however, and several individuals have been able to successfully stand against it, including Harry Potter, as well as both Bartemius Crouch Jr and Bartemius Crouch Sr, who learned to resist the curse after being subjected to its effects for an enduring period.

Imperare is Latin for "to order, command", and is the root of several modern English words, including imperative and emperor. Imperium means "(regarding) command" or "authority", and imperio is its ablative form, meaning (among other things) "with authority". Imperius is not, strictly speaking, a Latin word.