A swell, in the context of an ocean, is a formation of long wavelength ocean surface waves on the sea. Swells are far more stable in their directions and frequency than normal oceanic waves since they are formed by tropical storms and by stable wind systems.

Swells are often created by storms thousands of nautical miles away from the beach where they break. This distance allows the waves comprising the swells to become more stable, clean, and free of chop as they travel toward the coast. Waves generated by storm winds have the same speed and will group together and travel with each other, while others traveling at even a fraction of a meter per second slower will lag behind, ultimately arriving many minutes later due to the distance traveled.

Information on swell size and period is useful for surfers, as swells are generally more desirable to surf on than normal, locally-generated waves and chop. Swell size is typically the average height of the largest 33% of waves in a set, measured from the highest point of a wave (peak) to the lowest point (trough). Swell size is also known as the significant wave height and it is the cube of the significant wave height that typically is used as the important component of the equation to calculate how much energy that wave has to erode a coastline. Period is the average length of time between each wave in a set. The significant period is the average period between the third largest waves in a wave record.

Since swell-generated waves are mixed with normal sea waves, they can be difficult to detect with the naked eye (particularly away from the shore) if they are not significantly larger than the normal waves. From a signal analysis point of view, swells can be thought of as a fairly regular (though not continual) wave signal existing in the midst of strong noise (i.e., normal waves and chop).

Swells were used by Polynesians to maintain course when no other clues where available, such as on foggy nights.