The signature of a knot, σ(K), is equal to σ(V + Vt), the signature of V + Vt where V is a Seifert matrix for the knot and Vt is its transpose. The knot's signature also equals the signature of the intersection form on H2(W), where W is the two-fold branched cover of D4 over a pushed-in Seifert surface.
The signature provides bounds for the smooth 4-genus of a knot; for instance, slice knots have signature zero.
 Kauffman, L. and Taylor, L., "Signature of Links," Trans. Amer. Math. Soc. 216 (1976), 351-365.
 Rolfsen, D., Knots and Links, AMS Chelsea Publishing, Providence (2003).