What is ‘medical necessity’?

Imagine that we are considering whether our healthcare system (or insurer) should fund treatment or procedure X. One factor may be cited is of so-called ‘medical necessity’. The claim would X eligible for funding if it medically necessary, but ineligible this does not apply. Similarly, (and relevant to the debates in special issue), a particular ethically and/or legally permitted, wish distinguish between cases where and those were not. But what do mean by concept? Here I will propose briefly defend one plausible practical definition.