要件 is a formal term with two main senses depending on context. In legal and administrative settings it means an 'important document' such as a contract, certificate, or application form. In logic, law, or planning contexts it means an 'essential condition' or 'prerequisite' — something that must be true or present for something else to hold. Common in fixed phrases like 构成要件 ('constituent elements') and 必要条件 ('necessary condition').