What is Cestui Que Vie and Cestui Que Trust?

"The person who has a beneficial interest in a trust; the beneficiary."
https://law.academic.ru/9297/cestui_que_trust

"He who has a right to a beneficial interest in and out of an estate the legal title to which is vested in another."
https://thelawdictionary.org/cestui-que-trust/

"Cestui que vie is French for he who lives."
https://gtajatam.ga/business/how-to-access-cesta-que-trust.php

"IV If the supposed dead Man prove to be alive, then the Title is revested. Action for mean Profits with Interest."
http://www.legislation.gov.uk/aep/Cha2/18-19/11

"Definition of cestui que vie

1 : the person whose life measures the duration of an estate
2 : the person on whose life an insurance policy is written"
https://www.merriam-webster.com/dictionary/cestui%20que%20vie