From the Latin phrase; pro tempore. The preposition pro means "for" and tempore is the ablative of tempus meaning "time".
Pro-tem Sentence Examples
From 1882 to 1885 he was president pro tem.
He was president of New Hampshire in1785-1786and in 1788-1789; a member of the Federal Constitutional Convention in 1787, where he voted against granting to Congress the power of issuing paper money; a member of the state convention which ratified the Federal Constitution for New Hampshire; a member of the United States Senate in 178 9 -1801, and its president pro tem.