To startle is to jump, like when you're surprised. If you’re sneaking through a dark room and step on a sleeping dog’s tail — you might startle the dog. If she barks, that dog startles you right back!
A startle is a quick, sharp movement, like a little jump that happens when you’re surprised or suddenly scared. If you sneak up on your mom and say "Boo!" you’ll startle her and she might jump up in her seat (before she yells at you to quit it). Alarm clocks and barking dogs often startle people. The original meaning of startle, around 1300, was "to run back and forth," from the Old English word styrtan, "to leap up."