To simplify the description of the types of measurements a VNA can make, the reflection and transmission measurements are defined in terms of scattering parameters, or S‑parameters. For a 2‑port network, four fundamental S‑parameters can be measured, and they are defined as SXY. For a 2‑port VNA, measurements of signals leaving Port 1 are called forward measurements, and those leaving Port 2 are called reverse measurements. Signals that leave and return to the same port are designated reflection measurements, and those that leave one port and return to another port are designated transmission measurements. S‑parameters are an abbreviated designation for these measurements, and are used as shown in the following list:

The first number (X) in SXY is the port number in which the signal is being received, and the second number (Y) is the port number from which the signal is being transmitted. The S‑parameter is a ratio of these two signals.

Forward Reflection represents the measurement in which the incident signal is transmitted from port 1 and is reflected back to port 1.

Forward Transmission represents the measurement in which the incident signal is transmitted from port 1 and is received at port 2.

Reverse Transmission represents the measurement in which the incident signal is transmitted from port 2 and is received at port 1.

Reverse Reflection represents the measurement in which the incident signal is transmitted from port 2 and is reflected back to port 2.