An individual citizen of a state but so characterized by and while in transit, working, residing or otherwise in the territory of another state while not in receipt of any immigration status in the host state.
Also a foreign national while within the jurisdiction of another state.
The US Code, Title 8, Chapter 12, s. 1101 defines an alien as "any person not a citizen or national of the United States"
The Canadian constitutions gives to the federal government the legislative powers over "Naturalization and Aliens".
In The King v Hayes, the Ontario Court adopted these words:
"One born in a strange country under the obedience of a strange prince or country. To prove that persons are aliens it is not sufficient to swear merely that they are aliens, but particulars must be given to shew how they were aliens as by having been born in a certain place named out of the allegiance of the British Crown. An alien who came to Canada and after a residence of ten years took the oath of allegiance, but who had taken no proceedings to obtain a certificate of naturalization was held to be still an alien."
References and Further Reading:
The King v Hayes 1903 6 CCC 357