In a proportion with three terms, the mean term is the mean proportional between the two other terms.
The expression “mean proportional” is a synonym of “geometric mean“.
Example
Consider this proportion:
\(\dfrac{2}{x}\) = \(\dfrac{x}{8}\)
The term \(x\) is a mean proportional.
We can calculate it with the geometric mean :
\(\overline{x}_g\) = \(\sqrt[2]{2 × 8}\) = \(\sqrt[2]{16} = 4\)