wave equation
The PDE u_{tt} = c^2u_{xx} (or its higher-dimensional analog) modeling a disturbance that propagates at a fixed speed c.
The wave equation u_{tt} = c^2\nabla^2u (one spatial dimension: u_{tt}=c^2u_{xx}) describes a disturbance -- a string's displacement, a pressure wave -- that propagates through a medium at fixed speed c.
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