GNU Collaborative International Dictionary of English

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Found one definition

1.       
   Altern (), a. [L. alternus, fr. alter another: cf. F. alterne.] Acting by turns; alternate.  Milton.
   
   [1913 Webster]
   
   Altern base (Trig.), a second side made base, in distinction from a side previously regarded as base.
   
   [1913 Webster]