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A normal mode oscillates at one frequency with a fixed spatial eigenfunction.

Tension is a pulling force transmitted along a taut string, rope, or similar body. For an ideal uniform string with linear mass density $\mu$, constant tension $T$ sets the small transverse wave speed $c=\sqrt{T/\mu }$.

A stretched uniform string obeys y_tt=c^2 y_xx with wave speed equal to the square root of tension over density.

The wave speed $c$ is the propagation speed in a wave equation such as $u_{tt}=c^2{\nabla }^{2}u$.