A pennant is usually a narrow tapering flag most commonly flown by ships at sea.

A commissioning pennant is the traditional sign of a warship, and is flown from the masthead while the ship is in commission.

A broad pennant in the Royal Navy is a swallow-tailed tapering flag flown from the masthead of a ship to indicate the presence of a commodore. It is so called because its dimensions are roughly 2:3.

A church pennant in European navies is flown during church services. In the United States Navy, a pennant is also flown over the national colors during religious services.

Unlike the triangular pennant, most national and departmental flags are rectangular; the national flag of Nepal and the state flag of Ohio in the U.S. are the only notable modern flags to be a variation of the standard pennant.

In many localities within the United States, a pennant-shaped sign, yellow in color and marked with the words "No Passing Zone," is placed along a roadside, and denotes the start of a stretch of a two-lane highway in which one vehicle is not legally permitted to pass another traveling in the same direction. This is not, however, universal; other jurisdictions make use of a white, rectangular sign emblazoned with the words "Do Not Pass," for this purpose.

See also Maritime flags

In baseball, a pennant is a commemorative flag flown by the champion of a league, and has come to refer to the league championship itself. The "pennant race" is the last few weeks of the regular season (often expressed specifically as the month of September), when lower-ranked teams are steadily eliminated from contention, leaving the three divisional champions and one wild card team from each league at the beginning of the playoffs. The winners of the American League Championship Series and National League Championship Series are then each said to have won their league's respective pennant.