An object or person gets dismounted when they are taken off or removed from their usual place, like a horse that's no longer riding.
Imagine being on a bike, and then someone helps you take it off so you can put it away. You would be "dismounting" the bike, which means getting off of it. People also use this word when they're talking about horses - for example, if a rider gets down from their horse, we say they've dismounted.
The part of a routine in which the gymnast detaches from an apparatus.
Example: It was a stylish routine, let down by a sloppy dismount.
To (cause to) get off (something).
Example: She carefully dismounted from the horse.
