When two people whip out their smart phones and compare their recently downloaded apps in an attempt to determine which person has the coolest app. Usually done between someone owning an iPhone and an Android phone since there is a heated rivalry between the two mobile OSes. The only way an app battle ends is when one of the two participants battery runs out or the people around them get annoyed and leave.
Bro: "Hey check out this awesome new app i got yesterday. Only cost me 99 cents."
Me: "Not bad. Not bad. But i got this guy (officially the worlds coolest app) last week for free.
Bro: "Thats alright i guess. Look at this this one though! You shoot birds at these pig dudes"
And thusly the app battle began