For a vector with entries
The negative definite, positive semi-definite, and negative semi-definite matrices are defined in the same way, except that the expression zTMz or z*Mz is required to be always negative, non-negative, and non-positive, respectively.
Positive definite matrices are closely related to positive-definite symmetric bilinear forms (or sesquilinear forms in the complex case), and to inner products of vector spaces.[1]
Some authors use more general definitions of "positive definite" that include some non-symmetric real matrices, or non-Hermitian complex ones.