Formal:
* Mandatory: This is a close synonym that emphasizes the required nature of something.
* Compulsory: Similar to mandatory, but with a slightly stronger sense of force or pressure.
* Required: This is a more neutral term, simply stating that something is necessary.
* Prescribed: This word implies a specific rule or regulation that dictates the requirement.
* Imperative: This emphasizes the urgent or essential nature of the requirement.
Informal:
* Must: This is a straightforward and direct way to express obligation.
* Have to: Another common way to express obligation, slightly less formal than "must."
* Required: This is a simple and neutral word that can be used in many contexts.
* Essential: This word emphasizes the importance of the requirement.
* Needed: This highlights the necessity of the requirement.
The best word to use depends on the specific situation and the desired level of formality.