Virginity is pretty much a social term- it means something slightly different to every person that talks about it.
Traditionally, "virgin" means a person (though was used in reference to women more than men) who has engaged in sexual intercourse- that is, the penis has gone into the vagina. This of course is a very limited definition of sex, not taking into account, manual sex, oral sex, anal sex, and of course it's totally heterocentric, not to mention usually sexist in its application. According to this traditional definition, it is not something you can go back from by being celibate for any period of time.
In terms of how different religious traditions view virginity, you'd really have to consult the religious leaders of those traditions. I'm not sure what Catholics would say, but I know that many Christian groups believe that it is possible to have a "second virginity" after converting to that religious tradition- but they all differ, and I don't know what the view is of any non-Christian traditions. They also have different views on what constitutes sex- some define it as intercourse, while others define passionate kissing as a sexual activity. Many churches also say that a rape has no affect on a person's virginity.
Personally, I don't like the idea of virginity. As we can see from a quick look at the concept, it's not a well defined term, and to me, the concept it devalues sex, because it makes the "first time" seem like something precious to be held up on a pedastal- implying that any sex after that can never have the same value.
Miz Scarlet also wrote us an excellent article on this topic, going into they way in which "virginity" has been used as a means of oppression in the past:
Magical Cups & Bloody Brides