iOS, is short form for iPhone OS. i in iPhone, doesn't really stand for something (it's derived from iPod, which was derived from iMac). Accoding to wikipedia,

iOS, is short form for iPhone OS. i in iPhone, doesn't really stand for something (it's derived from iPod, which was derived from iMac). Accoding to wikipedia, Jobs said the "i" stood for "internet, individual, instruct, inform, and inspire". Given that the abbreviation isn't literal, "internet OS" is not colloquial and you will confuse others.