complex conjugate

Flipping the sign of a complex number's imaginary part -- $x^* = a - jb$ for $x = a+jb$ .

The complex conjugate of $x=a+jb$ is $x^*=a-jb$ -- same real part, imaginary part negated. Geometrically this reflects $x$ across the real axis. Conjugation shows up constantly in this course because $x x^* = |x|^2$ , a real, non-negative number, turns a complex quantity into an ordinary size.