To calculate the derivative of a real function \\(f(x)\\) using the limit definition, use the formula \\( f'(x) = \\lim_{h \ o 0} \\frac{f(x+h) - f(x)}{h} \\). This calculates the slope of the tangent to the point \\(x\\) on the curve, indicating the instantaneous rate of change of the function at that point.