In plasma physics, the Lundquist number (denoted by ) is a dimensionless ratio which compares the timescale of an Alfvén wave crossing to the timescale of resistive diffusion. It is a special case of the magnetic Reynolds number when the Alfvén velocity is the typical velocity scale of the system, and is given by
where is the typical length scale of the system, is the magnetic diffusivity and is the Alfvén velocity of the plasma.
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.