The Tolman-Oppenheimer-Volkoff limit (TOV or Landau-Oppenheimer-Volkoff limit or LOV) is the maximum mass of a neutron star, i.e., the mass at which gravitational pull is balanced with the outward degeneracy pressure. It is the analog of the Chandrasekhar limit that limits the size of white dwarves. It is not merely a solution to a single equation, but has been developed and improved over time as more about neutron stars has been modeled and discovered. Current values are in the neighborhood of 2.1 solar masses. Like the Chandrasekhar limit, it assumes absence of rotation (and possibly no magnetic field); rotation could plausibly allow a higher mass neutron star.
The limit was seriously theorized by Lev Landau in 1932, and worked on worked on by Richard Chace Tolman. J. Robert Oppenheimer and George Volkoff did further work in 1939, creating the Tolman-Oppenheimer-Volkoff equation (TOV equation) which is an equation for hydrostatic equilibrium that takes general relativity into account. Their value for the TOV limit was 0.7 solar masses, but using a relatively simple equation of state, modeling the neutron star as uniform. With further, more detailed neutron star models, the TOV limit has been thought to be within the 1.5 to 3.0 solar mass range, which would be the stellar remnant of a main sequence star in the 15 to 20 solar mass range. The recent advent of GW detections involving neutron stars has provided an additional source of information.