Having an online dictionary with you at all times is a terrific convenience. Google's "define" search feature is probably one of the quickest and most convenient ways to look up a word, but it's only good when you have an Internet connection. Providentially, there are two mobile apps that allow you to look up definitions even when you're offline.
The Merriam-Webster and Dictionary.com mobile apps are two of the most popular dictionary apps in the Google Play Store and the Apple App Store. The Android versions of the apps and Merriam-Webster's iOS app can all be used offline, which should come in handy when you're on a plane or in a remote location without Internet access.
The Merriam-Webster app (Android | iOS) automatically includes data files for offline access. For this reason, the app is rather large, over 100MB for the Android version. Keep in mind that audio pronunciations are only available when online.
The Dictionary.com Android app will prompt you after install to download the offline data files. If you're on a metered data plan, you may want to wait until you're on a Wi-Fi connection to download the files. Just like the Merriam-Webster app, pronunciations are only available when online. Synonyms are also only available when online.