Emittance is a property of a charged particle beam in a particle accelerator. It is a measure for the average spread of particle coordinates in position and momentum phase space and has the dimension of length times angle.

Emittance diagram.  The inner ellipse, containing about 39% of the beam current and called the beam core, is free of much of the beam aberrations and has a temperature of 1eV or  11600 Kelvin. The outer ellipse contains the entire beam and has a much higher temperature of around 50eV.