A computer software program. A "killer app" is an extremely good program. For example, e-mail was once considered the Internet's killer app, SMS was next, and now there are thousands of "apps" for smart phones.
Historical perspective: "There's an app for that," says Apple in a spate of ads for its popular iPhone smart phone. With a reported 100,000 apps as of late 2009, many of which also work on Apple's iPod Touch, some of these free and paid apps are extraordinary. With an app, you can easily make a dinner reservation, reserve an airplane seat, check your bank balance, read Excel spreadsheets, or play Skee-ball. In addition to those written for the iPhone, about 3,000 apps are available for Research in Motion's popular BlackBerry and about 10,000 for the line of phone that operate on the Google Android operating system. Some of the most popular apps are free. Shazam identifies a song you're listening to and helps you buy it online. Gas Buddy finds the cheapest fuel near your location. Many cable and satellite companies offer apps that let you remotely program your DVR from anywhere in the world. Other apps cost anywhere from 99 cents to a few dollars. Save Benjis compares product prices across a number of stores, Scrabble lets you play the classic game solo or with others through a Wi-Fi connection, and NetLingo let you look up every text and chat acronym you need to know!
The term "app" was listed as "Word of the Year" by the American Dialect Society in 2010.