switch terms
Described in [1] 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. Frequenctly 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.
[1]R.B. Marks, “Formulations of the Basic Vector Network Analyzer Error Model including Switch-Terms,” in ARFTG Conference Digest-Fall, 50th, 1997, vol. 32, pp. 115-126.