"Graduation" at the college and university level occurs when the presiding officer confers degrees upon candidates, either individually or en masse, even if graduates physically receive their diploma later at a smaller college or departmental ceremony. After degree completion, graduates can be referred to by their graduating year.
early 15c., in alchemy, "tempering, refining of something to a certain degree; measurement according to the four degrees of a quality," from graduate (n.). General sense of "dividing into degrees" is from 1590s; meaning "action of receiving or giving an academic degree" is from early 15c.; in reference to the ceremony where a degree is given, from 1818.