From the Latin "aqua", meaning "water", and "mens", meaning "mind". "Agua" is also Spanish for "water".
This spell was first seen in the Half-Blood Prince. It causes a jet of water to shoot from the end of the caster's wand. The speed of the water is able to be controlled as is the ammount. Harry had used it to refill a small goblet in the Horcrux cave after Dumbledore had drank the Emerald Potion.