What is Waqf (endowment) in Islamic law, and what are its fields?

"Waqf" linguistically means "detention" or "holding back." In Islamic law, it refers to a perpetual endowment made by a competent owner of his usable property, while retaining its corpus, cutting off his and others' ability to dispose of its ownership through any type of transaction. This endowment's proceeds are directed towards a charitable cause, seeking proximity to Allah the Almighty. It is recommended, based on the Prophet's (peace be upon him) saying: "When a person dies, his deeds cease except for three: a continuous charity (Sadaqa Jariya), knowledge from which others benefit, or a righteous child who supplicates for him." The endowment of any asset that can be lawfully sold and from which lawful benefit can be derived while its corpus remains intact is valid, such as real estate, animals, and weapons. This is supported by two hadiths: "As for Khalid, he has endowed his shields and equipment for the sake of Allah," and the hadith (definition): "Whoever stables his horse for the sake of Allah, out of faith and seeking reward, then its satiation, dung, and urine will be weighed as good deeds in his balance."