淫乱 means lewdness or sexual depravity, often describing a person's character, while 淫蕩 focuses on the act or lifestyle of debauchery.
Etymology
Compound of 淫 (いん, 'lewd, excessive') and 蕩 (とう, 'melt, be charmed, dissipated'). The exact historical derivation is uncertain, but the combination conveys the idea of being morally melted or led astray by sensual pleasures.
