Flexible Spaced Recognition
This is a far more flexible app than Memrise or similar: you can fix your cards, or add and delete them right from the app itself, so it is right there when you come across a new word or concept. In addition, card setup is highly customisable, albeit perhaps tricky if you dont know CSS/HTML. It may seem expensive, but it is highly worthwhile in my opinion.
SteveMassey about
AnkiMobile Flashcards, v2.0.19