Semi-definite and Gramian matrix

It is called positive-semidefinite (or sometimes nonnegative-definite) if

A matrix M is positive-semidefinite if and only if it arises as the Gram matrix of some set of vectors. In contrast to the positive-definite case, these vectors need not be linearly independent.

Gram matrix:

In linear algebra, the Gramian matrix (or Gram matrix or Gramian) of a set of vectors  in an inner product space is the Hermitian matrix of inner products, whose entries are given by .

http://en.wikipedia.org/wiki/Positive-semidefinite_matrix#Negative-definite.2C_semidefinite_and_indefinite_matrices