diglyph

from The Century Dictionary.

• noun In architecture, an ornament consisting essentially of two associated cuts or channels. Compare triglyph.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Arch.) A projecting face like the triglyph, but having only two channels or grooves sunk in it.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A member with two vertical channels or grooves, without the two lateral half grooves which characterize the triglyph.