NTR or Nuclear thermal rocket is a subtype of rocket propulsion wherein gas/fluid is heated inside a special-purpose nuclear reactor and then expelled at large velocity.
Nuclear thermal rockets have a higher performance than traditional chemical rockets, despite not heating the propellant more. This is achieved because you can choose a propellant freely, allowing the use of low molar mass molecules like $H_2$. A NTR has higher efficiency, but less thrust, making it most useful for upper-stages or space tugs. No NTRs are currently operational, but prototypes were developed by both the US and the Soviet union. See NERVA and RD-0410.