A word invented in 1864 by Karl Ulrichs* to refer to a male who is sexually attracted to other males. He first used the term in his Vindex: Social and Legal Studies on Man-Manly Love, the first in a series of twelve books on the subject. He believed that homosexual males (urnings**) were born urnings, and should not be prosecuted for their inborn nature. The science of the day held that the human embryo contained two 'germs', one male and one female. One would win over and express itself in the fetus. Sometimes nether would win out over the other, and the child would be a hermaphrodites. Ulrichs theorized that in some cases the male germ would win over control of the body, but the female germ would win over the mind. In these cases, the female mind would be attracted to 'real men', whom he called dionings.
Ulrichs got greater response than he expected -- and a surprising amount of it was positive. Unfortunately, his theory couldn't account for all the positive responses he got. He had to modify it to account for lesbians (which he called urningin), and for males who did not feel that they were feminine (he changed his classification to a continuum -- mannling {very masculine}, urning, and weibling {very effeminate}). One problem he never really solved was that of bisexuals (uranodionings) - how could one person have two (whole) minds? (One male and one female). Even if he couldn't quite figure out the science, Ulrichs' work was very important socially, helping to found the homosexual emancipation movement in Germany.
Now we just say gay.
* Ulrichs was German, and hence these words sound German. They were also used by English speakers, until the psychiatrists replaced them with words like 'homosexuals' and 'inverts'.
** The word Uranian is also sometimes used. I haven't been able to find out if Ulrichs used this word himself. I also haven't been able to find out much about the use of the word uranian to mean homosexual, although it is worth noting that Uranus was a male god (or pregod) who was castrated.