Wiktionary
n. Any of various mountaineering maneuvers in which a climber who has fallen and is sliding down a slope halts the slide without recourse to a rope or other belay system.
Wikipedia
Self-arrest is a movement employed in mountaineering, in which a climber who has fallen and is sliding down a snow or ice-covered slope arrests the slide by themselves without recourse to a rope or other belay system. Self-arrest can be performed by using ice axe and a combination of a climber's boots, hands, feet, knees and elbows. Use of an ice axe greatly increases the probability of effectively stopping a fall down a snow field, ice field, or glacier.
Usage examples of "self-arrest".
If one of us slipped or caught his crampons on a rock rather than ice, it was up to that person to stop his slide by self-arrest with his ice axe.
At least until teenagers looking for a thrill and adults in a serious hurry found that nine times out of ten, one could keep the sledfoils in the groove just by glissading -- that is, by using one or more ice axes in the self-arrest position and keeping the speed low enough to stay in the trough.