The United States Immigration Laws have strict provisions regarding immigrants entering and/or residing the country. By definition, an illegal alien is any person in the United States that does not have the proper authority to enter or reside within the country.
Therefore, legally speaking, illegal aliens are not allowed to live in the United States. However, it is no secret that illegal aliens currently reside in the United States, reaching numbers in the millions. Being an illegal alien in the United States is against the law, though it is often times extremely difficult to determine which individuals are in fact illegal aliens.
Many illegal aliens in the United States carry on fairly regular lives, working to provide for the families and attending school like regular American citizens. Many times, illegal aliens will obtain falsified documents allowing them to obtain a driver's license and employment. Therefore, even though it is illegal, it is possible for an illegal alien to live in the United States.