Mediant

In the context of JI ratios, the mediant M of two ratios a/c and b/d is M=(a+b)/(c+d). It will always be between the two ratios (a/c < M < b/d, assuming a/c < b/d).

Wikipedia article on the mediant

also see:

The Noble Mediant: Complex ratios and metastable musical intervals, by Margo Schulter and David Keenan