Minkowski writes in his paper on Time and Space:
If, for simplicity, we retain the same zero point of space and time, the first-mentioned group signifies in mechanics that we may subject the axes of x,y,z at t = 0 to any rotation we choose about the origin, corresponding to the homogeneous linear transformations of the expression $$ x^2 + y^2 + z^2$$
I've done a Google search on "homogeneous transformation", which has only returned results that give examples such as rotation, scaling etc. What's its general defintion?