A knot which can be deformed into itself but with the orientation reversed. The simplest noninvertible knot is 08-017. No general technique is known for determining if a Knot is invertible. Burde and Zieschang (1985) give a tabulation from which it is possible to extract the invertible knots up to 10 crossings.
See also Amphichiral Knot
References
Burde, G. and Zieschang, H. Knots. Berlin: de Gruyter, 1985.