hydrate

If you drop an aldehyde or ketone into water, a little of it quietly reacts with the water itself. The water molecule adds across the C=O bond, and the product is a hydrate, a carbon bearing two -OH groups on the same carbon. Chemists call this arrangement a geminal diol (gem-diol), 'geminal' meaning the two like groups are twins on one atom.

Mechanistically it is plain nucleophilic addition: water's oxygen (the nucleophile) attacks the carbonyl carbon, the C=O electrons go onto the oxygen, and after proton shuffling you have HO-C-OH. The reaction is an equilibrium, and for most ordinary aldehydes and ketones it lies far to the carbonyl side, so only a tiny fraction is hydrated at any moment. But the position of the equilibrium depends strongly on the carbonyl: formaldehyde is almost completely hydrated in water, and trichloroacetaldehyde forms a stable, isolable hydrate (chloral hydrate) because the electron-withdrawing chlorines make the carbonyl extra hungry for the nucleophile.

Hydrates are usually a side note rather than a synthetic goal, because gem-diols are typically unstable and revert to the carbonyl plus water. But they are an honest, simple template for the whole nucleophilic-addition family: hemiacetals (with an alcohol instead of water) and the rest are just the same dance with a different partner.