符号函数（Sign function，简称sgn）是一个逻辑函数，用以判断实数的正负号。为避免和英文读音相似的正弦函数（sine）混淆，它亦称为Signum function。其定义为：

In mathematics, the sign function or signum function (from signum, Latin for "sign") is an odd mathematical function that extracts the sign of a real number. In mathematical expressions the sign function is often represented as sgn.