Is there a collective term for the XY, YZ and ZX planes in 3D co-ordinate geometry? I was thinking "principal planes" but I'm not sure where I heard that.
"Principal planes" (and principal axes, principal projections) are terms used in engineering, architecture and technical drawing when using orthogonal projections (usually several of them) to represent a 3-dimensional image. Names of other fields where the same words could arise: descriptive geometry, perspective drawing, computer graphics, axonometry, surveying, geodesy, artistic perspective.
In mathematics, "coordinate planes" is completely standard, as can be seen from the first comment and its multiple upvotes.
Apart from being non-standard, use of "principal" to refer to the coordinate axes or planes is imprecise, and it conflicts with a web of other (useful because mutually consistent) terms such as principal axes of an ellipsoid or of a stress, principal component analysis in statistics, principal diagonal of a matrix. One wants to keep the latter terms as they are because they all are expressing the same circle of concepts related to diagonalization of quadratic forms. So "coordinate planes" it is.