What are Semidefinite matrices?

Hi.
I have read that when dealing with 2x2 matrices, a positive definite matrix is one with a positive determinant and trace, and a negative definite matrix is one with a positive determinant and negative trace.

But what are semidefinite matrices? I understand that they can have 0 determinant/trace, but does that mean a matrix with, for example a determinant of 8 and a trace of 4, is both positive semidefinite AND definite, or just one or the other? The definitions seem to be overlapping and since I'm having to answer a multiple choice paper, with allowance for more than one answer per question, I want to get the definition down. :s-smilie: