Necessity

Different words for “necessary” in philosophy include essential condition, requirement, and terms that specify the type of necessity, such as metaphysical (could not be otherwise), logical (following from the laws of logic), nomological (following from the laws of nature), epistemic (known or knowable), moral, and practical. These terms are used to describe conditions that must be met, truths that must hold, or beings that must exist in all possible circumstances.
Common synonyms

Essential condition: A condition that must be present for something else to occur.
Requirement: Something that is needed or obligatory.
Mandatory: Something that is required by law or rule.
Indispensable: Absolutely necessary; crucial.

Terms specifying the type of necessity

Metaphysical necessity: A truth that holds in all possible worlds; it could not have been otherwise.
Logical necessity: A truth that must be true according to the laws of logic; it is necessarily true.
Nomological necessity: A truth that must be true according to the laws of nature and physics.
Epistemic necessity: A claim that is necessarily true because it is impossible to know otherwise.
Practical necessity: Something that is necessary for a specific purpose, goal, or action.
Moral necessity: Something that is required by morality or ethics. [1, 2, 4, 5, 8]

Other related concepts

Contingency: The opposite of necessity; something that could be otherwise.
Necessitarianism: The view that everything that is possible is also actual.
Necessary being: A being that exists in all possible worlds and cannot fail to exist.

AI responses may include mistakes.

[1] https://plato.stanford.edu/entries/modality-varieties/

[2] wikipedia/en/Metaphysical_necessity