This app was mentioned in 3 comments, with an average of 4.00 upvotes
Aedict Reader is a paid resource, but it's pretty much the best thing ever.
When you highlight text and copy it aedict reader is run and will lookup the definition.
Nazeka does that with instant popups on android firefox and is free.
Kaku is still alpha but it does that anywhere on android for free.
Aedict reader popup is now very pricey but is really well made, and is a floating semi transparent popup bubble with lots of features.
Imiwa is available. There is also a recent plugin for AEdict 3 on Android that does this
https://play.google.com/store/apps/details?id=sk.baka.aedictreader