For bounce juggling, a drop is a failure to touch a ball which, as a result, hits the ground twice in a row or hits any foreign object.
(a drop; is; a failure to touch a ball) (a ball; hits twice; L:in a row) (a ball; hits; any foreign object)
a drop a ball a ball
is hits twice hits
a failure to touch a ball
L:in a row
any foreign object