Let’s start with, what is the title? In case you didn’t know, Camus was French; so he wrote The Stranger in French, and because it seemed appropriate, gave it a French title: L'Étranger. Here’s where things get tricky – in the translation. "L'Étranger" could have easily been translated as "The Foreigner" instead of as "The Stranger," and actually is in some cases. Translations aside, it’s more fun to argue semantics. Let’s run with this "foreigner" bit. Our main character, Meursault, is a French man living in French Algiers. In some senses, yes, this makes him a foreigner to the land, but the text establishes that in fact his family has lived there for several generations. Viewed in this light, Meursault is no more a foreigner to French Algiers than third generation Americans are foreigners in the U.S. More likely, Meursault is a metaphorical foreigner. We know this guy is detachment personified, so it’s easy to argue that he’s a foreigner to society, to common, human customs – he’s an outsider (yet another possible translation for the title, by the way).
This is based on the word "foreigner," but the same thing applies to the title "The Stranger." Meursault is a stranger among other people because he is so isolated from them – mentally, emotionally, spiritually, and, by the end of the text, physically (he’s imprisoned). He’s strange. He’s a stranger. Or quite possibly, he’s the stranger.