Because a free H+ ion is technically a bare proton, acids are sometimes referred to as "proton donors" because they release hydrogen ions in solution. The term "proton donor" is misleading, since in aqueous solution, the hydrogen ion is never a bare proton- it's covalently bound to a water molecule as an H3O+ ion. Further, acids don't "donate" protons; they yield them to bases with a stronger affinity for them.