tribrach

from The Collaborative International Dictionary of English v.0.48

Tribrach \Tri"brach\, n. [L. tribrachys, Gr. ? consisting of
   three short syllables; ? (see {Tri-}) + ? short.] (Gr. & L.
   Pros.)
   A poetic foot of three short syllables, as, m[^e]l[^i][^u]s.
   [1913 Webster]