In simple terms, impedance, in a coaxial product, is the measurement of resistance to the flow of current. The unit of measurement is Ohms.

The following is a more technically correct definition:

Transmission line impedance, also known as characteristic impedance, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along an infinitely long transmission line with absence of any reflections. Characteristic impedance measures like resistance when dealing coaxial cable types. Characteristic impedance is a relationship between the capacitance per unit length and the inductance per unit length. The inner and outer coaxial diameter ratios and the dielectric constant in the cable define the parameters involved in determining characteristic impedance.