Swath width refers to the strip of the Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

’s surface from which data are collected by a satellite

Satellite

In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

. The longitudinal

Longitude

Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....

 extent of the swath is defined by the motion of the satellite with respect to the surface, whereas the swath width is measured perpendicularly to the longitudinal extent of the swath. Note that swath width is also cited in context of sonar

Sonar

Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...

specifications. It is measured and represented in metres or kilometers.