The Collaborative International Dictionary of English v.0.48:

Tenuis \Ten"u*is\, n.; pl. Tenues. [NL., fr. L. tenuis fine,
   thin. See Tenuous.] (Gr. Gram.)
   One of the three surd mutes [kappa], [pi], [tau]; -- so
   called in relation to their respective middle letters, or
   medials, [gamma], [beta], [delta], and their aspirates,
   [chi], [phi], [theta]. The term is also applied to the
   corresponding letters and articulate elements in other
   languages.
   [1913 Webster]