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.
Knot 
, illustrated above, is a ribbon knot (Rolfsen
1976, p. 225).
