illegal alien or illegal immigrant = ?
someone or a Mexican who enters the US illegally is called what?
"Illegal immigrant" is more formal and specific. Alien is a broader word that includes anyone foreign to a particular place (including, of course, creatures foreign to earth).
Generally the two terms are used interchangeably, though technically an illegal immigrant would be someone who has actually moved or immigrated to a place, whereas an illegal alien could be anyone who is currently in the country illegally.
Of course then you have "undocumented immigrant" or "undocumented worker". These are used mostly by people who defend illegal immigration/immigrants. Sometimes people will also just use the word "immigrant" when they really mean illegal immigrant to downplay the whole breaking the law aspect.
I've never understood the "alien" part of illegal alien. Illegal immigrant is better, which is also the description that suits you if you enter illegally into Japan, Chile, Australia, Canada, Mexico, etc.
Depends on how you feel about the situation. Around here (southern California) you hear indocumentado a lot (undocumented one). That's short for inmigrante indocumentado (illegal/undocumented immigrant).
You can say it either way. But I hear illegal immigrant more where I live (Northern Virginia). The "alien" part of illegal alien is used to say that they are foreign but I rarely hear illegal alien used.
I am sure that illegal immigrant is a much clearer description since the word alien suggests that they come from a different planet - like something out of a Stephen Spielberg film - wheras in fact they are still human and simply living or seeking to live in a country illegally and whatever the rights and wrongs of that, they are still human.
(One further thought: the Big trouble with such terms as immigrant or illegal immigrant is that they are very emotive/evocative and so can be used to stir up hatred against such people and No... I am not one of them, nor do I defend their illegal behaviour. Like statistics these terms can be used to obscure the reality that these people are human beings like us with families to feed. I would just hate to be in their shoes since so many people in this world like to treat them like dirt...including politicians who make political capital out of them. By the way, I am not for one minute suggesting that anyone here is seeking to do that but pointing out the dangers that the media and politicians frequently harp on about the problems perhaps if the latter were to work with the leaders of other nations to help provide work for themin their own countries they wouldn't need to go looking for work elsewhere)
Inmigrante ilegal, though in Spanish it's usually heard "Vive en USA pero está ilegal"