- switch terms
Described in  by Roger Marks, switch-terms account for the fact that the 8-term (aka error-box ) model is overly simplified. The two error networks change slightly depending on which port is being excited. This is due to the internal switch within the VNA.
- port impedance
The impedance of a given port on a microwave Network. Frequently this is the same as the characteristic impedance of a given transmission line mode, but may be arbitrarily set. terminating a port with any impedance other than its port impedance will cause reflections.
- characteristic impedance
Ratio of voltage to current, for a single traveling wave along a transmission line.