A petition is a request to an authority, most commonly a government official or public entity. In the colloquial sense, a petition is a document addressed to some official and signed by numerous individuals. A petition may be oral rather than written, and in this era may be transmitted via the Internet. The term also has a specific meaning in the legal profession as a request, directed to a court or administrative tribunal, seeking some sort of relief such as a court order.

The Petition Clause of the First Amendment to the U.S. Constitution guarantees the right of the people "to petition the Government for a redress of grievances." The right to petition has been held to include the right to file lawsuits against the government.

Petitions are commonly used in the U.S. to qualify candidates for public office to appear on a ballot; while anyone can be a write-in candidate, a candidate desiring that his or her name appear on printed ballots and other official election materials must gather a certain number of valid signatures from registered voters. In jurisdictions whose laws allow for ballot initiatives, the gathering of a sufficient number of voter signatures qualilfies a proposed initiative to be placed on the ballot. The 2003 California recall election, which culminated in the recall of Governor Gray Davis and the election of Arnold Schwarzenegger, began when U.S. Representative Darrell Issa employed hundreds of paid signature gatherers who obtained millions of signatures at a cost to Issa of millions of dollars. Once the requisite number of signatures was obtained on the recall petition, other petitions were circulated by would-be candidates who wanted to appear on the ballot as possible replacements for Davis. After that step, a vote on the recall was scheduled.

Other types of petitions have included those which sought to free Nelson Mandela during his imprisonment by the former apartheid government of South Africa. The petitions had no legal effect, but the signatures of millions of people on the petitions represented a moral force which may have helped to free Mandela and to end apartheid. Non-governmental organization such as Amnesty International often use petitions in an attempt to exert moral authority in support of various causes.

A petition can also be the title of a legal pleading that initiates a case to be heard before a court. The initial pleading in a civil lawsuit that seeks only money (damages) might be titled (in most U.S. courts) a complaint; an initial pleading in a lawsuit seeking non-monetary or "equitable" relief such as a request for a writ of mandamus or habeas corpus, or for custody of a child or for probate of a will, would instead be termed a petition.