Immoral, unmoral, nonmoral, amoral are all briefly definable as not moral, yet they are not often interchangeable and are frequently confused, largely because the implications and connotations of the second element are not the same in each compound.
Immoral, which implies an active opposition to what is moral, may designate whatever is discordant with accepted ethical principles or the dictates of conscience and then indicates that what is so designated is fundamentally wrong, unjustifiable, or sinful. But, like moral, immoral may often base its values not on principle but on custom and then may imply no more than discordance with accepted social custom or the general practice.
In its frequent specific application to sexual and, especially, irregular sexual matters immoral tends to fluctuate between the two extremes of its usage range according to the concurrent rigidity of the social outlook. It may in the former case come close to licentious or lewd in pejorative quality and in the second lose most of its pejorative force and mean little more than improper or immodest. Unmoral, nonmoral, and amoral all, in contrast to immoral, imply in one way or another a passive negation of what is moral especially as indicated by absence of or freedom from a code that ought to prevail and the evasion of which constitutes wrongdoing.
In its most typical use unmoral implies a lack of moral perception and ethical awareness and is appropriately applied to persons or to their behavior when these exhibit such a lack; thus, an infant or an idiot may be described as unmoral because in neither case is there a capacity to distinguish right from wrong.
But unmoral may sometimes imply a mere disregard of or failure to be guided by moral principles and is then close to conscienceless and occasionally it may, along with nonmoral and amoral, imply that what is so qualified cannot be appraised in terms of morality since it is not a fit subject for ethical judgment.
While nonmoral and amoral are frequently interchangeable, nonmoral may be preferred when the thing so described is patently outside the sphere in which moral distinctions or judgments are applicable, while amoral may be applied discriminatively to something not customarily or universally exempted from moral judgment; thus, life in the abstract is a nonmoral concept although a particular personal life may well be amoral; perspective is a nonmoral aspect of painting.