Most applications on the Apple App Store are "universal" apps, which means they will work on any iOS device. While you can't technically transfer an app you downloaded onto your iPod touch or iPhone to your iPad, all the apps you have purchased can be re-downloaded to any of your iOS devices. The easiest way to "transfer / copy" apps to your iPad is to connect it to the same iTunes account as your other devices and download the app again. You can find a list of apps you have purchased under the Purchased tab in the Updates section of the App Store app. Just tap the cloud icon next to the name of the app to begin downloading it to your iPad.
How do I transfer an app from my iPod touch / iPhone to my iPad
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