The symbol used to denote the differential operator. So if y is a real function of x then $y\prime =\frac{\text{d}y}{\text{d}x}$ is the differential of y with respect to x. Since it is $\frac{\text{d}}{\text{d}x}\left(y\right)$, when the second differential is taken, it is written as $\frac{{\text{d}}^{2}y}{\text{d}{x}^2}$ coming from $\frac{\text{d}}{\text{d}x}\left(\frac{\text{d}y}{\text{d}x}\right)$.

See also exterior derivative.

Abbreviation for deci-.