User blog comment:AlthaBlade/Bloons Wiki News - November 17, 2012/@comment-5199397-20121117010008/@comment-3153244-20121117031425

Actually, if I have understood it correctly, iOS is exclusive to Apple. In simpler terms, the lowercase i is like the iPhone, iPod, etc, while OS means Operating System. So together, Bloons TD 5 iOS kinda means BTD5 for Apple Operating System.