The Labor aristocracy, in Marxist-Leninist theory, is a category of workers (proletarians) in developed countries, who benefit from the superprofits extracted by the capitalist ruling classes of their countries from the impoverished workers of underdeveloped countries.
In other words, thanks to imperialism, the ruling classes of developed nations are able to exploit workers abroad and raise the living standards of their own workers back home, thus buying the loyalty of those workers and avoiding a potential revolution. The workers whose loyalty is bought by the capitalists in this manner form the "labor aristocracy", and they will generally try to defend the capitalist system.
Due to the existence of a labor aristocracy in the developed countries, Lenin argued that a workers' revolution could only begin in one of the underdeveloped countries, such as Russia.
See also: Leninism