the president of the United states rejects a bill passed by Congress he is demonstrating what principle

He is demonstrating the principle of veto.

Veto comes from Latin 'I forbid'. The U. S President has the veto power to refuse to approve a bill or a resolution passed by Congress. The President has ten day (without Sunday) to sign a particular bill. A veto occurs when the President returns the bill without signing it into law. He returns the bill to the House from which it originated, together with an explanation for his veto.

The President's veto can be overridden by a two-third vote in both the Senate and the House. If this happens, the bill becomes law despite the veto.

It is the principle of veto