Why are acids called proton donors?

Acids are called proton donors because they have a tendency to lose, or donate, H+ atoms to bases when they are dissolved in water or other solvents. This concept is based on the Brownsted-Lowry theory, which states a Brownsted-Lowry acid is any species that is capable of donating a proton (H+). For example, when an acid dissolves in water, it donates a proton to a water molecule, producing hydronium ions (H3O+). Any Brownsted-Lowry acid must contain a hydrogen that can dissociate as H+.