Dummy Variable
A variable that appears in a calculation only as a placeholder and which disappears completely in the final result. For example, in the integral integral_0^x f(x') d x', x' is a dummy variable since it is "integrated out" in the final answer. Any variable name other than x' could therefore be used in the above expression, e.g., integral_0^x f(λ) d λ, integral_0^x f(q) d q, etc.