Algebraic Number Theory
《Algebraic Number Theory》 · 第 417 页 · Chapter VI，Section 8「The Reciprocity Law of the Power Residues」，定义段
The symbol $\left(\frac ab\right)$ is called the Jacobi symbol, or also the quadratic residue symbol (although, for $b$ not a prime number, the condition that $\left(\frac ab\right)=1$ is no longer equivalent to the condition that $a$ is a quadratic residue modulo $b$).