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.

Back to your original question - BTD5 won't be released on Android for a while. Someone (Tosh or Aaron) mentioned that BTD5 will be released on Android or HD (i.e. iPad) within 3 weeks. (Can't remember which one - Android or iPad)