If the Knot is the boundary of a singular disk which has the property that each self-intersecting component is an arc for which consists of two arcs in , one of which is interior, then is said to be a ribbon knot. Every ribbon knot is a Slice Knot, and it is conjectured that every Slice Knot is a ribbon knot.
See also Slice Knot
References
Rolfsen, D. Knots and Links. Wilmington, DE: Publish or Perish Press, p. 225, 1976.