A quantity that determines the extent to which thermionic or photoelectric emission will occur according to the Richardson equation or Einstein’s photoelectric equation. It is sometimes expressed as a potential difference (symbol φ‎) in volts and sometimes as the energy required to remove an electron (symbol W) in electronvolts or joules. The former has been called the work function potential and the latter the work function energy.

http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/photoelec.html Values given on Carl Nave’s HyperPhysics site at Georgia State University