This app was mentioned in 5 comments, with an average of 3.40 upvotes
Haven't tried them all, but to me Dictionary by The Free Dictionary is the best. Mostly because it lets you download the dictionaries to use them offline, a feature I haven't seen much in other apps.
Thanks, I gave it a try.
Given, that almost none of the features are implemented yet, I'd be a bit hesitant calling this an open beta - it appears to be in very early alpha stages of development.
Apart from that, the biggest issue I'm having with it so far is the dedication necessary to manually enter the entire content of this app on a mobile keyboard. I can't see many people putting up with that - I know I won't.
You should significantly focus on eliminating that tedious process as much as possible if you want people to actually use the app - so adding import tools and online sources will be vital for this app.
Luckily that shouldn't be too hard to do, given that there are excellent and free online sources readily available:
Once you implement the ability to look up words on Wiktionary and save the word and its definition (and optionally even Wiktionary's tags) inside the app with just a few clicks, it would make the app infinitely more useful and actually solve the problem you are trying to solve with it ("I often found myself reading an article or watching an interesting video when I hear a word that sounds great, but that I don't know the meaning of.").
Additionally, allow automatic import of Wiktionary's word of the day and/or it's archive or certain categories of words.
Later you can focus on adding other online resources as long as they allow free access for commercial applications.
Remember that people will want to use the app in more than just English and possibly multiple different languages at the same time, so multiple word-lists are vital - the tagging feature should be able to take care of that, though.
The app needs to be a tool to look up, save and study words in order to be an efficient tool. While you are clearly planning to implement the "study" part, the look up part is IMHO even more important to make this a useful tool.
And given that the official Wiktionary app has been abandoned and the competition (e.g.) is ugly, outdated or plain terrible - you might actually have a winning app at your fingertips. But only if you shift the current focus massively towards word discovery via online sources.
The free dictionary allows you to download dictionaries and use offline https://play.google.com/store/apps/details?id=com.tfd.mobile.TfdSearch
The Free Dictionary is the best I've used and one of the few that allow offline function.