Moving in or forming a straight line. Also, characterized by straight lines. "Rectilinear" comes from the late Latin "rectilineus," which itself comes from the Latin words "rectus" ("straight") and "linea" ("line"). "Rectilineal" is a synonymous form of this word. Because "rectilineal" remains far less popular than its relative, writers might as well avoid it unless there's a need to rhyme with anneal.(pr. REK-tə-LI-nee-ər and REK-tə-LI-nee-əl)Also see lattice, linear, linear perspective, rectangle, and rectangular.A term that is distantly related to rectilinear is "rectitude," the formal, dignified bearing adopted by proctologists.