The California Appellate Court, Third Division
Judges Sims, Raye and Hull, said in a decision:

http://caselaw.lp.findlaw.com/data2/californiastatecases/c054124.pdf

“Defendants prefer the term “undocumented immigrants.” However,defendants do not cite any authoritative definition of the term and do not support their assertion that the terms “undocumented immigrant” and “illegal alien” are interchangeable. We consider the term “illegal alien” less ambiguous. Thus, under federal law, an “alien” is “any person not a citizen or national of theUnited States.” (8 U.S.C. § 1101(a)(3).) A “national of the United States” means a U.S. citizen or a noncitizen who owes permanent allegiance to the United States. (8 U.S.C. § 1101(a)(22).) Under federal law, “immigrant” means every alien except those classified by federal law as nonimmigrant aliens. (8 U.S.C. § 1101(a)(15).) “Nonimmigrant aliens” are, in general, temporary visitors to the United States, such as diplomats and students who have no intention of abandoning their residence in a foreign country. (8 U.S.C. § 1101(a)(15)(F), (G); Elkins v. Moreno (1978) 435 U.S. 647, 664-665 [55 L.Ed.2d
614, 627-628] [under pre-1996 law, held the question whether nonimmigrant aliens could become domiciliaries of Maryland for purposes of in-state college tuition was a matter of state law].) The federal statutes at issue in this appeal refer to“alien[s] who [are] not lawfully present in the United States.” (8 U.S.C. §§ 1621(d), 1623.) In place of the cumbersome phrase“alien[s] who [are] not lawfully present,” we shall use the term “illegal aliens.” “

Also see here