The French title of Camus' work, called The Stranger in most English translations, is L'Étranger. This has been variously translated as The Estranged One, The Outsider, and The Foreigner. These translations do have opposing nuances of meaning. The Stranger suggests social isolation. The Foreigner suggests cultural difference. The Outsider suggests the personal behavior of an idiosyncratic person who acts in such a way as to be set apart from others. The Estranged One suggests one who has had a natural relationship severed.
You can see that each option holds a different light to the main thematic element of the story. My personal preference is for The Outsider as it relates to a person's behavior and its results, which is what the novel is most about: How Meursault acts in an absurd world and the consequences of his actions, this in a world where even the Sun behaves absurdly while at one time being helpful and at another time being destructive.
With this said so as to give a deeper perspective into Meursault as a character and Camus as a novelist, we'll examine Meursault as a stranger. It is difficult to argue that he is a stranger to himself since he is so keenly aware of every sensation he has and so bitingly honest and direct about his desires and intentions and emotional feelings:
he asked me a last question: Did I regret what I had done?
After thinking a bit, I said that what I felt was less regret than a kind of vexation—I couldn’t find a better word for it.
Yet, it may be said he is a stranger to himself if one takes the position that morality and emotional empathy are innate qualities (something author William Golding contests). In this light, he is a stranger to himself because he is isolated from his social obligations and moral duties, as was demonstrated at the vigil for his mother.
It is easier to argue that he is a stranger to society in that he does not hold with, believe in, follow with society's traditions, rules, mores, or expectations. For example, he does not mourn his losses since he doesn't feel them other than intellectually. He does not love with yearning, which he reveals by explaining that he would agree to marry any girl he liked and who might ask him:
I said I didn’t mind; if she was keen on it, we’d get married. ... she asked me again if I loved her. I replied, ... I supposed I didn’t. ... but, if it would give her pleasure, we could get married right away. ...
“Suppose another girl had asked you to marry her—I mean, a girl you liked in the same way as you like me—would you have said ‘Yes’ to her, too?”
Again, it is harder to argue that he is a stranger to his environment since it is his environment that he feels so keenly and that influences him so profoundly: "the glare of the morning sun hit me in the eyes like a clenched fist." Yet it might be argued that he is a stranger to his environment in that he has no way within his coping devices to control or mitigate the raw effect of his environment upon himself. In other words, it might be said that had he not been a stranger to his environment, he would have known and understood more fully the impact the sun and heat and glare and hot wind had upon him and taken measures to protect himself from his environment.
I was conscious only of the cymbals of the sun clashing on my skull, ... scarring my eyelashes, and gouging into my eyeballs. ... a fiery gust came from the sea, ... a great sheet of flame poured down through the rift.