Chisini mean
In mathematics, a function f of n variables x1,..., xn leads to a Chisini mean M if, for every vector ⟨x1,..., xn⟩, there exists a unique M such that
The arithmetic, harmonic, geometric, generalised, Heronian and quadratic means are all Chisini means, as are their weighted variants.
While Oscar Chisini was arguably the first to deal with "substitution means" in some depth in 1929, the idea of defining a mean as above is quite old, appearing in early works of Augustus De Morgan.