It's a rather new Japanese word meaning 'a person
absorbed mostly in computer at home.'
I wonder what is the English word for that.
Yes, that would be a "nerd".
"Geek" is almost synonymous with "nerd", but has a slightly less pejorative tone. A geek might describe himself as a "geek"; but even a nerd would never describe himself as a "nerd".
It is (just) possible to be a "girl geek"; but on the whole, "nerd" is only applied to males.
Thank you very much, Mr. Pedantic.