Dictionary.net

• tr[=oo]-m[=o]', n. any piece of wall between two openings:--pl. TRUMEAUX' (-m[=o]z'). [Fr.]

1908 - Chambers's Twentieth Century Dictionary of the English Language
By Thomas Davidson

• (archit., pl. -eaux). Piece of wall, pillar, between two openings, e.g. pillar dividing large doorway.

1919 - The concise Oxford dictionary of current English
By Sir Augustus Henry