the GNU version of the Collaborative International Dictionary of English
adj. Three sided, the sides being plane or concave; having three salient angles or edges; trigonal.
The Century Dictionary and Cyclopedia
Three-sided; triangular; having three plane or concave sides
Word Usage
"The spikelets are plano-convex, orbicular to oblong, obtuse, secund, 2-ranked on the flattened or triquetrous rachis of the spike-like branches of a raceme, one-flowered and falling off entire from the very short or obscure pedicels."