1) Concept in physics 2) Wave mechanic 
Historically, the problem of a vibrating string such as that of a musical instrument was studied by Jean le Rond d`Alembert, Leonhard Euler, Daniel Bernoulli, and Joseph-Louis Lagrange. In 1746, d’Alembert discovered the one-dimensional wave equation, and within ten years Euler discovered the three-dimensional wave equation. == Introduction == ....
(from the article `analysis`) ...in terms of formulas, this meant that the shapes of the curves were defined by different formulas in different intervals. In 1749 he went on to ... ...fields, and Maxwell introduced the idea of `displacement current` to make the set of equations logically consistent. As a result, he found on .....
[n] - a differential equation that describes the passage of harmonic waves through a medium
The wave equation describes the propagation of waves.wherec = wave velocity
noun a differential equation that describes the passage of harmonic waves through a medium