APP could mean:

• Amyloid precursor protein, a protein fragment associated with some neurological disorders
• Asia Pulp & Paper
• Associated Press of Pakistan
• The Alan Parsons Project, a British progressive rock band
• Application software, Slang for an executable program used on a computer

In Political Parties

• Anticlerical Progress Party REASON, a Polish political party
• Austrian People's Party, an Austrian political party
• American Populist Party, an American political party

In Railway Code

• Amtrak's 3-letter code for the rail station at Appleton, Wisconsin, USA
• The National Rail code for Appleby railway station, United Kingdom.

APP