Definition
The veto power is the president's constitutional authority to refuse to sign a bill passed by both houses of Congress, sending it back instead of letting it become law. It is set out in Article I, Section 7.
The veto is a defining feature of the system of checks and balances. It lets the executive branch participate in lawmaking without granting it the power to legislate, since Congress retains the final say through an override.
Why it matters
How it works
When Congress sends a bill to the president, several outcomes are possible. The president may sign it into law, or return it with objections, which is a regular veto. Congress can then attempt an override, which requires a two-thirds majority in both the House and the Senate.
If the president neither signs nor returns the bill within ten days while Congress is in session, it becomes law automatically. But if Congress adjourns during that window, the bill dies through a pocket veto, which cannot be overridden. The president cannot strike individual lines, so the choice is to accept or reject the whole bill.