deduction (n.)

early 15c., deduccioun, "a bringing, a leading;" mid-15c., "action of deducting; a taking away, a number or amount subtracted," from Old French deduction (Modern French déduction) and directly from Latin deductionem (nominative deductio) "a leading away, an escorting; a diminution," noun of action from past-participle stem of deducere "lead or bring away or down; derive" (in Medieval Latin, "infer logically"), from de "down" (see de-) + ducere "to lead," from PIE root *deuk- "to lead."

Meaning "that which is deducted" is from 1540s. As a term in logic, "derivation as a result from a known principle, an inference, conclusion," 1520s, from Late Latin use of deductio as a loan-translation of Greek apagoge. Related: Deductional.