In electrical engineering, admittance is a measure of how easily a circuit or device will allow a current to flow. It is defined as the reciprocal of impedance, analogous to how conductance & resistance are defined. The SI unit of admittance is the siemens (symbol S); the older, synonymous unit is mho, and its symbol is ℧ (an upside-down uppercase omega Ω). Oliver Heaviside coined the term admittance in December 1887. Heaviside used Y to represent the magnitude of admittance, but it quickly became the conventional symbol for admittance itself through the publications of Charles Proteus Steinmetz. Heaviside probably chose Y simply because it is next to Z in the alphabet, the conventional symbol for impedance.

Admittance is defined as

${\displaystyle Y\equiv {\frac {1}{Z}}\,}$

where

Y is the admittance, measured in siemens

Z is the impedance, measured in ohms

Resistance is a measure of the opposition of a circuit to the flow of a steady current, while impedance takes into account not only the resistance but also dynamic effects (known as reactance). Likewise, admittance is not only a measure of the ease with which a steady current can flow, but also the dynamic effects of the material's susceptance to polarization:

${\displaystyle Y=G+jB\,}$

where

• ${\displaystyle Y}$ is the admittance, measured in siemens.
• ${\displaystyle G}$ is the conductance, measured in siemens.
• ${\displaystyle B}$ is the susceptance, measured in siemens.
• ${\displaystyle j^{2}=-1}$

The dynamic effects of the material's susceptance relate to the universal dielectric response, the power law scaling of a system's admittance with frequency under alternating current conditions.