Modern trust law prefers the word beneficiary but the older cases refer to the older Latin term of cestui que trust.
In Green v Underhill, Justice Romer adopted these words:
"A trust is an equitable obligation binding a person (who is called a trustee) to deal with property over which he has control (which is caled the trust property) for the benefit of persons (who are called beneficiaries or cestui que trust) of whom he may himself be one, and anyone of whom may enforce the obligation. Any act or neglect on the part of a trustee which is not authorised or excused by the terms of the trust instrument, or by law, is a breach of trust."
REFERENCES:
Green v Underhill 2 QB 226 (1959)