Kronecker Delta
From Conservapedia
The Kronecker Delta 
satisfies the following property:
where 
and 
are integers. For a summation:
.
The elements of the identity matrix can be seen as following Kronecker Delta (i.e., 
). To see this, let 
be an nxn matrix, 
be its elements and 
be the nxn identity matrix. Then 
so,
where the definition of matrix multiplication and the above property of summation was used.
The continuous analogue of Kronecker Delta is Dirac delta.
References
Weisstein, Eric W. "Kronecker Delta." From MathWorld—A Wolfram Web Resource. http://mathworld.wolfram.com/KroneckerDelta.html
