The Shrinking Charm is a spell that causes an Engorged object to return to its normal size.
References from the canon
"redusen" Middle Eng. diminish, from "reducer" Old Fr. bring back to the source, from "reducere" L. bring, lead
The incantation "reducio" is used by Harry to cast the Reductor Curse at one point in the books (GF11). However, Harry describes the effect of the Reductor Curse as being to "blast solid objects out of his way" (GF31).
He uses that spell in the Maze during the Third Task:
Harry approached it cautiously, pointing the wand's beam at it. This looked like some kind of enchantment. He wondered whether he might be able to blast it out of the way.
"Reducio!" he said.
The spell shot straight through the mist, leaving it intact. He supposed he should have known better; the Reductor Curse was for solid objects. What would happen if he walked through the mist? Was it worth chancing it, or should he double back?
Perhaps the reason that the spell didn't work wasn't because he was aiming it at a non-solid object but because he used the wrong incantation!