For a few years, I’ve been learning biblical Hebrew and Greek vocabulary with Bible Vocab by Robert Turnbull. Turnbull also has a similar app for other languages called Vocab Pro. Like Vocab Pro, Bible Vocab allows you to easily create decks of cards from popular textbooks. But the Bible Vocab app also let you create decks based on any biblical passage; this may be one verse or the whole Bible. You can also customize your decks according to word frequency.
But what makes this app really useful is the spaced repetition feature. Here’s how it works. You are shown a word for the first time with it’s gloss(es). Then, five seconds later, you see it again without the gloss. If you get it right, then it’s 30 seconds, then 15 min, then an hour, and so on. If you get it wrong, the card drops back to the beginning or not so far back if you choose. The introductory video explains it best. Automatic spaced repetition is perfect for solidifying your vocabulary and would be impossible to replicate with paper flash cards. I’m not always in favor of screen tech over paper tech but this time the app wins.