签署 means 'to sign (formally)' — typically used for official documents such as contracts, treaties, agreements, or legal papers. It emphasizes the solemn, binding nature of the act: signing is not casual but carries legal or institutional weight. The character 签 (qiān) originally meant 'to mark with a signature' or 'to endorse', while 署 (shǔ) means 'to sign one's name' or 'to affix a signature'; together, they reinforce formality and authority.

This verb is commonly used in business, diplomacy, government, and academic contexts. It’s almost always transitive — it requires a direct object (e.g., a contract, agreement, or declaration). Unlike the more general 签 (qiān), which can mean 'to sign' in everyday situations like signing a receipt, 签署 is reserved for high-stakes, official acts and rarely appears in informal speech or personal contexts.