You searched for: “a ringlike band of muscle fibers that constricts a passage or closes a natural orifice”
A unit related to: “a ringlike band of muscle fibers that constricts a passage or closes a natural orifice”
sphinctero-, sphincter- +
(Greek > Latin: that which binds tightly, press together; band, lace; hence, muscle that closes an aperture of the body; a ringlike band of muscle fibers that constricts a passage or closes a natural orifice)