a scaling, flaking, or discoloring skin condition that made a person ritually unclean and shut out of the community.
Despite the traditional English translation, this word does not mean "leprosy" in the modern medical sense (the nerve-destroying illness now called Hansen's disease). It is a broad, eye-based category for skin that flakes, scales, or turns patchy and pale. The Greek word grew out of an older family of words about peeling and husking, and it named what a condition looked like on the surface, not a single germ or diagnosis. Greek doctors of the time even used a separate word for the disfiguring disease we now call leprosy, and they never called that one by this name.
In the Gospels the real issue is not medicine but purity. The category comes straight from the law of Moses, where a priest inspected such a person and declared them clean or unclean, and an unclean person had to live apart from everyone else. That is why all four Gospel writers say a person with this condition is made clean rather than healed: the deepest problem was being cut off from worship and community, and being made clean meant being allowed back in.
The same underlying idea in the Hebrew Bible could even be applied to mold or decay on clothing and on the walls of a house, which shows how far the term reaches beyond any one disease. Read this way, the word points to a kind of visible decay on the boundary between life and death, and to the social and religious shut-out that came with it, more than to a specific illness.