A steadiness of mind that holds especially under pressure.
A steadiness of mind that holds especially under pressure — an even keel that fortune's swings rarely manage to upset.
Mental evenness defined by staying balanced under pressure: it “suggests a habit of mind that is only rarely disturbed under great strain.” This is its key contrast with composure, which “implies the controlling of emotional or mental agitation by an effort of will” — equanimity is undisturbed inner equilibrium, where composure can be effortful management of agitation present underneath. Unlike serenity, tranquility, or calmness (peace without a disturbing force), equanimity specifically presupposes pressure it remains even against. At home in Stoic and Buddhist contexts.
c. 1600 “fairness, impartiality,” from Latin aequanimitas “evenness of mind,” from aequus “even, level” + animus “mind” (cf. aequo animo “with even mind”).
Entered meaning “fairness of judgment” (now obsolete); the sense “evenness of temper” is from the 1610s, later extended to general balance and harmony. The shift runs judgment-fairness → evenness under stress → general balance. No reliable recent-generation shift.