d^2B/dD^2 represents the second derivative of B(t) with respect to D(t). In this context, it measures how the rate at which births occur changes as deaths occur.
Related terms
First Derivative: Also known as the derivative, it measures how a function changes with respect to its independent variable.
Differentiation: The process of finding derivatives of functions.
Rate of Change of Rates: A measure of how the rate at which something is changing itself changes over time.