The Collaborative International Dictionary of English v.0.48:

Tern \Tern\, a. [L. pl. terni three each, three; akin to tres
   three. See Three, and cf. Trine.]
   Threefold; triple; consisting of three; ternate.
   [1913 Webster]

   Tern flowers (Bot.), flowers growing three and three
      together.

   Tern leaves (Bot.), leaves arranged in threes, or three by
      three, or having three in each whorl or set.

   Tern peduncles (Bot.), three peduncles growing together
      from the same axis.

   Tern schooner (Naut.), a three-masted schooner.
      [1913 Webster]