Gaussian Coordinate System
A coordinate system which has a metric satisfying g_(ii)=-1 and partialg_(ij)/partialx_j=0.
Explore with Wolfram|Alpha
WolframAlpha
More things to try:
Cite this as:
Weisstein, Eric W. "Gaussian Coordinate System." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/GaussianCoordinateSystem.html