Reef Reef, v. t. [imp. & p. p. Reefed (r[=e]ft); p. pr. & vb. n. Reefing.] (Naut.) To reduce the extent of (as a sail) by roiling or folding a certain portion of it and making it fast to the yard or spar. --Totten. To reef the paddles, to move the floats of a paddle wheel toward its center so that they will not dip so deeply.