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How to say " 宅男 "

according to the Japanese definition it's called "Otaku"

but if you want to borrow the Western terminology I could refer to the terms "couch potato" or "mouse potato", implying a person who remains sedentary for the entire day or simply stays at home and remains immobile.

It depends on the context. You can never find a one word translation for this kind of words.

This word came from Japanese, originally お宅 or 御宅(おたく, pronounced otaku). If you wanna use this word in it's original Japanese meaning, then use "otaku",as hakee suggested, which means social incompetent people, normally obsessed with electronics and Japanese animation, comics, and games(hereinafter ACG). Lately some otakus petitioned to legalize the marriage with ACG characters. The protagonists in the Japanese drama "秋葉原@deep" are stereotype otakus, though exaggerated.

There are words of similar meaning in western culture, "nerd"(more in Britain) and "geek"(more in US), which also mean social incompetent people, sometimes very into electronics, sci-fi, superhero comic and similar hobbies (traditionally US ones, but it's gradually changing by the significant influence of Japanese ACG). Nevertheless, the aforementioned terms "nerd" and "geek" often possess the sense of "weird genius"(at least knowledgeable). Stereotype of nerds and geeks(again, exaggerated) can be found in the sitcoms "the IT crowd" and "big bang theory" respectively.

By the way, I'm one of them.
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Obviously my essay is driving me mental...

Hikikomori (from Japanese)
a shut-in (singular)
shut-ins (plural)

Dimitri is right.
I agree that Taiwan's definition is totally inconsistent with its Japanese source.
I read a book called "Shutting Out the Sun" one year ago by Michael Zielenziger, an American journalist in Japan, and he gave a detailed description of the Japanese hikikomori phenomenon and its main possible cause.

The Japanese government refused to admit that the phenomena could be a kind of mental disorder until early this century. Now hikikomori can seek professional medical help.

According to Shutting Out the Sun, most hikikomoris are bullied at school when they were kids and Japanese communities look down on the minority, the bullied. Even the government denied their existence. Those who are not accepted by the Japanese rigid weird traditional values confine themselves to avoid people's look. This phenomenon is very unique in Japan and some of those people recover somehow once they go abroad. Some hikikomoris are so afraid to go out that they don't have any haircut for one or two years and some of them are even violent to parents.

The book was absorbing and informant. Books of social sciences are always my favorite.

Otaku