Curvilinear is a modern Latinate compound of Latin curvus ('curved') + linea ('line'), formed on the model of 'rectilinear'. The ancestry passes through Latin curvus and Latin linea. In present use as an adjective, it means "consisting of or bounded by curved lines", keeping the historical formation close to the word's ordinary semantic work. As a compound, its meaning is assembled from the two visible parts rather than from an opaque borrowing. Related words include curve, curvature, and linear; derived forms include curvilinearly and curvilinearity.