Horizontal phase velocity
= Horizontal phase velocity
{title2=$c_x=\omega/k$}
For phase $kx+mz-\omega t$ with $k\ne0$, hold $z$ fixed: a contour of constant <wave phase> advances in $x$ at $\omega/k$. This is the speed at which a tilted <wavefront> crosses a horizontal line. It differs from the horizontal component of the vector <phase velocity>, $\omega k/(k^2+m^2)$, which is defined along the <wavevector>.