A lowercase Greek letter commonly representing small changes, variations, or partial derivatives in calculus and physics.
Examples
Change in a variable
\delta x = x_2 - x_1
Small variation in calculus
\delta f = f(x + \delta x) - f(x)
Kronecker delta function
\delta_{ij} = \begin{cases} 1 & \text{if } i = j \\ 0 & \text{if } i \neq j \end{cases}