An n × n real symmetric matrix M is positive definite if zTMz > 0 for all non-zero vectors z with real entries ( ), where zT denotes the transpose of z.
An n × n complex Hermitian matrix M is positive definite if z*Mz > 0 for all non-zero complex vectors z, where z* denotes the conjugate transpose of z. The quantityz*Mz is always real because M is a Hermitian matrix.