The Arithmetic of Elliptic Curves
《The Arithmetic of Elliptic Curves》 · 第 42–43 页 · Chapter III，Section 1「Weierstrass Equations」，Definition
For a Weierstrass equation, put $j=c_4^3/\Delta$. […] The quantity $\Delta$ is the discriminant of the Weierstrass equation, the quantity $j$ is the $j$-invariant of the elliptic curve.