Kronecker delta

noun2

1. •
   A binary function, written as δ with two subscripts, which evaluates to 1 when its arguments are equal, and 0 otherwise.

   • The permutation symbol and the Kronecker delta prove to be very useful in proving vector identities. — 2007
2. •
   A unary function, written as δ with a single index, which evaluates to 1 at zero, and 0 elsewhere.