Trail running is a variant on running

Running

Running is a means of terrestrial locomotion allowing humans and other animals to move rapidly on foot. It is simply defined in athletics terms as a gait in which at regular points during the running cycle both feet are off the ground...

 that differs markedly from road running

Road running

Road running is the sport of running on a measured course over an established road . These events would be classified as long distance according to athletics terminology, with distances typically ranging from 5 kilometers to 42.2 kilometers in the marathon. They may involve large numbers of runners...

 and track running. Trail

Trail

A trail is a path with a rough beaten or dirt/stone surface used for travel. Trails may be for use only by walkers and in some places are the main access route to remote settlements...

 running generally takes place on hiking trails, most commonly single track trails, although fire roads are not uncommon. A distinguishing characteristic of the trails is that they are often inaccessible by road except at the trail heads. The trails tend to traverse varying terrain; hills, mountains, deserts, forests, and narrow passages are common.