Definition of "couplet"

A set of two things, particularly

1. (literature) A pair of lines, typically with rhyming end words.  examples

2. A pair of one-way streets which carry opposing directions of traffic through gridded urban areas.  examples

3. (taxonomy) A pair of two mutually exclusive choices in a dichotomous key.  quotations  examples

   Quotations

   • The dichotomous keys are constructed so that each couplet presents a set of alternative choices.

     2001, Stephen T. Ross, The Inland Fishes of Mississippi, page 33

   • As long as the correct statement of each couplet is chosen, and the unknown organism is included in the key, a confident identification is usually achieved.

     2004, Shin'ichiro Ishikawa, An Exploration of a New Poetic Expression Beyond Dichotomy