A catalyst is a substance that promotes a reaction but is not used up by the reaction process. So, a catalytic substance can continually promote reactions without having to supply additional catalyst.

In the stratosphere, a catalytic cycle of ozone destruction exists where a catalyst "X" peels off an oxygen atom from an ozone molecule, then gives it up to a stray oxygen atom that hasn't yet found an oxygen molecule to attach to (which would have formed ozone). The leftover catalyst "X" can then initiate a new destruction cycle. The net result is the same as the second of the ozone destruction reactions in the Chapman Cycle.