The story
From the mathematical formula -(∂Δ)/(∂τ), the partial derivative of delta (Δ) with respect to time (τ), pronounced as "D delta (by) D time".
Etymology from Wiktionary, CC BY-SA 4.0.
DdeltaDtime
Today it means: A second-order measure of derivative price sensitivity, expressed as the instantaneous rate of change of delta with respect to time. Full definition →