Dictionary.com
Thesaurus.com

monoplane vs. biplane

[mon-uh-pleyn] / ˈmɒn əˌpleɪn /

noun

  1. an airplane with one main sustaining surface or one set of wings.

  2. Nautical. a planing plane craft the bottom of which is in an unbroken fore-and-aft line.

[bahy-pleyn] / ˈbaɪˌpleɪn /

noun

  1. an airplane with two sets of wings, one above and usually slightly forward of the other.