1640s, "making or exhibiting a difference," from Medieval Latin differentialis, from Latin differentia "diversity, difference" (see difference). Related: Differentially. As a noun in mathematics, "an infinitesimal difference between two values of variable quantity," from 1704. Differential calculus is attested from 1702.
late 12c., "a feudal superior, ruler possessing the fealty of other rulers," from over- + lord (n.). In English history, especially a king of one of the Anglo-Saxon realms who held pre-eminence or authority over some of the other kings and chiefs. The word was chosen in 1943 as the Allied code-word for the invasion of western Europe that resulted in D-Day.
In the perspective of 'Overlord,' that one huge hazardous offensive operation on which, it seemed, the fate of the world depended, smaller adventures receded to infinitesimal importance. [Evelyn Waugh, "Unconditional Surrender"]