An object representing the eventual result of an async operation. It is a placeholder for a future value. A promise has three states: pending, fulfilled, and rejected. It settles (fulfills or rejects) exactly once.

Pending (initial, not yet settled), fulfilled (operation succeeded, has a value), and rejected (operation failed, has a reason/error). A promise transitions from pending to fulfilled or rejected once and cannot change state after settling.

Use new Promise((resolve, reject) => { ... }). The executor function runs immediately. Call resolve(value) to fulfill the promise, or reject(reason) to reject it. The executor usually starts an async operation.