I'm playing around with the define axis command, and noticed that in the documentation the link to "plane-spec" does not lead to anything that defines how to specify a plane (help:user/commands/atomspec.html#othermodels). Or at least, I can't see it. The link to "point-spec" does take me to an informative page (help:user/commands/pointspec.html#point).

Is the syntax for specifying a plane defined elsewhere? 

This is with 1.6.1.

Cheers,

Alexis