Dirac delta

Etymology

 * the notation was chosen because the symbol behaves analogously to the Kronecker delta.

Noun

 * 1)  A function-like symbol denoted as &delta;, with the property $$\int f(t)\delta(x-t) \mathrm d t = f(x)$$, informally understood as a function with an infinite value at 0 and zero elsewhere, and formally as a distribution or a measure.