Extravagance, n.
Etymology: [Cf. F. extravagance. See Extravagant, and cf. Extravaganza.]
1. A wandering beyond proper limits; an excursion or sally from the usual way, course, or limit.
2. The state of being extravagant, wild, or prodigal beyond bounds of propriety or duty; want of moderation; excess; especially, undue expenditure of money; vaid and superfluous expense; prodigality; as, extravagance of anger, love, expression, imagination, demands. Some verses of my own, Maximin and Almanzor, cry vengeance on me for their extravagance. Dryden. The income of three dukes was enough to supply her extravagance. Arbuthnot.
Syn. -- Wildness; irregularity; excess; prodigality; profusion; waste; lavishness; unreasonableness; recklessness.
Shades of meaning
Fernald has no entry of its own for extravagance; he lists it under enthusiasm, excess.
Where he writes about it:
The old meaning of enthusiasm implies a pseudo-inspiration, an almost frantic extravagance in behalf of something supposed to be an expression of the divine will. — from enthusiasm
Careless expenditure in excess of income is extravagance; we may have also extravagance of language, professions, etc. — from excess
As extravagance is excess in outlay, exorbitance is excess in demands, and especially in pecuniary demands upon others. — from excess