Having a dictionary installed on your device is almost necessary. Even if English is your mother tongue, sometimes you need a dictionary too look up jargon or even to write an official letter!
One of the best dictionaries in play store is Merriam Webster Dictionary which is very easy to use with prediction and a very hand voice search which unfortunately only works when you have internet.
Merriam Webster Dictionary is one of the most famous dictionaries in the world and this reputation is well earned. It can be a very useful app for students, language learners, merchants and others.