Nihilism is, basically, the belief in little or nothing. Originally the word was used to attack accused heretics through the middle ages. As time passes, however, the term became put on a specific branch of philosophy, a radical sort of skepticism retaining the nonexistence of any objective basis for fact. Out of this standpoint they demanded the complete rejection of most established views and establishments, being constructed on a basis of subjectivity.
Meursault, in Camus' "The Stranger," is apparently the quintessential nihilist. A guy mentally at chances with the complete universe, he plods through life understanding no certainties and looking after nothing. Despite certain characteristics in keeping with existentialism, another philosophical school posting nihilism's rejection of established views, but putting focus on responsibility and participation in life, it should be said that as a result of Meursault's denial of objective real truth, and rejection of laws and regulations and institutions, he's first and foremost a nihilist.
The first & most significant pointer to Meursault's nihilistic tendencies is normally his denial of any objective basis for fact. This fairly hard to ascertain, because because of the ambiguous aspect of the narrative, very little is in fact established as 'true'. Nonetheless it is however an obvious feature of Meursault, as could be established through his frame of mind toward his own existence. After all, that lifestyle itself exists, and is purposeful, is normally one thing that practically everyone considers to be authentic. But Meursault treats seemingly crucial decisions with great indifference, and even takes little if any interest in anything. If nothing exists in fact it is impossible to learn anything (because anything you think you understand is totally subjective), why bother doing anything?