In plasma physics, the Lundquist number (denoted by ) is the dimensionless ratio of an Alfvén wave crossing timescale to a resistive diffusion timescale. In SI units, it is given by
where is a typical length scale, is the permeability of free space, is the Alfvén speed for a plasma with magnetic field , density , ion mass and is the plasma resistivity. High Lundquist numbers indicate highly conducting plasmas, while low Lundquist numbers indicate more resistive plasmas. Laboratory plasma experiments typically have Lundquist numbers between , while in astrophysical situations the Lundquist number can be greater than . Considerations of Lundquist number are especially important in magnetic reconnection.