  Dirac delta function of matrix argument is employed frequently in the
development of diverse fields such as Random Matrix Theory, Quantum Information
Theory, etc. The purpose of the article is pedagogical, it begins by recalling
detailed knowledge about Heaviside unit step function and Dirac delta function.
Then its extensions of Dirac delta function to vector spaces and matrix spaces
are discussed systematically, respectively. The detailed and elementary proofs
of these results are provided. Though we have not seen these results formulated
in the literature, there certainly are predecessors. Applications are also
mentioned.
