What you need to know about Sunday's debate over the NSA's surveillance program

What you need to know about Sunday's Senate debate

Patriot Act

Legislation signed into law by President George W. Bush on Oct. 26, 2001, in the wake of the Sept. 11 terrorist attacks by Al-Qaida. The bill authorized enhanced domestic security measures to prevent terrorism, including new surveillance powers.

Section 215

The part of the Patriot Act that the National Security Agency says gives it the authority to gather Americans' phone data. The Patriot Act amended Section 215 of the Foreign Intelligence Surveillance Act of 1978 to allow intelligence agencies to query phone data.

Sunset

A provision or clause in a statute or regulation that requires that the law will expire after a specific date unless further legislative action is taken to extend it. Three Patriot Act provisions are set to expire at midnight on Sunday unless they are extended.

USA Freedom Act

Legislation passed by the U.S. House of Representatives that would extend the three expiring provisions but rein in current surveillance initiatives by ending the so-called bulk collection program. It fell three votes short of Senate passage before Memorial Day.

Filibuster

A tactic used in the Senate to block a vote. A senator can speak indefinitely to delay or prevent a vote; to stop a filibuster, 60 of the 100 senators must vote to end it. Sen. Rand Paul, R-Ky., is expected to launch a filibuster Sunday to preclude a Freedom Act vote.