[French, He or she who.] The person for whom a benefit exists.
A cestui que trust is a person for whose benefit a trust is created; a beneficiary. Although legal title of the trust is vested in the trustee, the cestui que trust is the beneficiary who is entitled to all benefits from a trust.
A cestui que use is an archaic term of property law that describes one who has a beneficial interest in land held by someone else. Title and possession as well as the duty to defend the land is held by another, but the cestui que use has the right to rents, profits, and other benefits from the land.
A cestui que vie is the person whose life is used to measure various things, such as the duration of a trust, a gift, or an insurance contract. It can also be used to mean the person upon whose life a policy of life insurance is drawn.