The term "terminal planes" come up very frequently in books on microwave circuits. What exactly does the term mean?
The closest answer I have got is
A terminal plane, or reference plane, is the equivalent of a terminal pair in a low-frequency network -Foundations of microwave engineering, Robert E Collins
But I don't understand what a "plane" is in this sense, and I don't understand why we couldn't just use the term terminal port?
It's called a reference plane because, for both coaxial cable and hollow rectangular waveguides, an ideal connector cuts across the guide in a plane cut.
This is what a waveguide connector looks like, a flange with a plane cut across the guide.
This is a pair of APC-7 (A Precision Connector - 7mm) connectors, designed for precision measurement specifically so that the breaks in the inner and the outer line up to form a plane.
Distances from the 'connector' are measured from this plane.
In a more conventional RF connector like BNC, N-type or SMA, the break in the outer defines the plane.
Any plane across the guide can be taken as a reference plane when doing maths with the phase of RF waves propagating along the guide.
The most usually used planes are those of the connectors, as we are usually interested in reflection coefficients of components with respect to their connectors.
You will however sometimes see evaluation boards for RF components, with a line drawn on the board across a microstrip line, with a note that some S-Parameters are quoted with reference to this plane. This can be useful when you will be copying the reference design, and can choose that cut point as your virtual connector.
The reference plane is exactly that: The plane relative to which distances, and hence phases, etc are described.
This often goes a bit beyond the strict meaning of the word "plane": it often also comes with an (usually very explicitly stated) understanding that a wave front "comes in" through that plane, and that the plane is at a given reference impedance \$Z_0\$. Think of it as a "cut" through the path a wave travels, especially in closed wave guides.
This becomes especially relevant with measurement equipment, where "phase obtained through a system" really has no meaning until you specify at which point you define your zero-phase to be.
