"the idea that the more any given resource is applied to production, the lower the marginal gain in output, until a point is reached where the additional inputs produce no additional output, is referred to as: "

The idea that the more any given resource is applied to production, the lower the marginal gain in output, until a point is reached where the additional inputs produce no additional output, is referred to as: the law of diminishing returns.
According this law if one factor of production is increased while other factors are held constant (are held constant, the output per unit of the variable factor will eventually diminish. Example would be changing the number of workers and keeping the same machines and workspace.