The inverse of impedance. Usually denoted as "Y" in electrical equations, or 1/Z, Z being impedance. The unit value of admittance is commonly dubbed "mhos" (backwards spelling of ohms.) It is a measure of the lack of impedance in a circuit, system of circuits or a specific conductor. A short circuit should, ideally, have an admittance of infinity. An open circuit will have an ideal admittance of 0 mhos.
Z = 10 ohms
Y = 1/Z = 1/10 = 0.1 mhos
Student 1: What's admittance?
Student 2: I don't know. I just use 1/Z.
Student 1: Oh ok.