Scratch

A horse is a scratch when injury or other necessity causes it to be withdrawn from the race.

Bobble/Stumble

A slip or a duck from a horse trying to leave the starting gates, often caused by the surface giving way under the feet. This can range from a mild misstep to a stumble down to their knees.

Trifecta

A wager picking the first, second and third horses in the exact order. A trifecta box means paying more to ensure the horses can finish in any order in the top three spots.