Etymology. The term dichotomy is from the Greek language Greek: διχοτομία dichotomía "dividing in two" from δίχα dícha "in two, asunder" and τομή tomḗ "a cutting, incision".. Usage and examples. In set theory, a dichotomous relation R is such that either aRb, bRa, but not both.; A false dichotomy is an informal fallacy consisting of a supposed dichotomy which fails one or ...