Why You Forget Every Word You Look Up
You are reading something good and you hit a word you do not know. You look it up. The definition makes perfect sense. You feel that small click of understanding and you carry on reading. Four days later the same word turns up somewhere else and you have absolutely no idea what it means.
That is not you being bad at vocabulary. That is the ordinary behaviour of human memory and it happens to everyone who reads anything difficult. The interesting part is why, because once you know why, the fix is fairly obvious and it is much smaller than you would think.
Understanding something is not the same as remembering it
When you read a definition you are recognising, not recalling. Recognition is cheap. The word is right there in front of you, the definition is right underneath it and your brain has to do almost nothing to connect the two. It feels like learning because it feels easy and clear.
Recall is the expensive one. That is you, four days later, with only the word in front of you and no definition anywhere, having to produce the meaning from nothing. You never practised that. You practised the easy version once and then closed the tab.
This is the single biggest reason vocabulary lists do not work. Reading down a list of fifty words and their meanings feels productive and it is almost entirely recognition. You finish the list feeling like you know fifty words. You know none of them in the way that matters, which is having one arrive unprompted in a sentence somebody says to you.
The forgetting curve, without the science lecture
Hermann Ebbinghaus worked this out in the 1880s by memorising nonsense syllables and testing himself at intervals, which is a fairly bleak way to spend your thirties. What he found holds up. Forgetting is steepest immediately. You lose a big chunk of new material within the first day and then the curve flattens out. Most of what you are ever going to forget, you forget almost straight away.
The other half of what he found is the useful half. Every time you successfully recall something the curve gets flatter. The first review buys you a few days. The next one buys you a couple of weeks. After four or five well-spaced encounters a word stops needing maintenance and becomes something you simply know.
So the number of encounters matters much less than their spacing. Five encounters in one evening is cramming and it is nearly worthless a fortnight later. Five encounters spread across a month is how a word becomes permanent. This is the entire idea behind spaced repetition and it is why Anki works for the people who stick with it.
Why the moment of encounter matters
There is a second thing going on that the memory literature is less loud about and I reckon it matters just as much in practice.
A word you look up arrives at the worst possible moment. You are mid-sentence in something else. Your attention is on the argument of the paragraph, not on the word. The word is an obstacle between you and the thing you actually wanted to read, so you clear it and move on as fast as you can. You gave it three seconds and none of your attention.
Compare that with a word that arrives on its own, at a time you picked, when you are not in the middle of anything. You have room for it. You read the definition properly. You read the example sentence and think about whether you would ever say that. Sometimes you say it out loud, which feels silly and works. That is thirty seconds of genuine attention instead of three seconds of impatience.
The words I actually own are almost all from that second category. Not the ones I looked up because they were in my way.
What actually works
Three things, in order of how much they buy you for the effort.
Space the encounters out. One word a day beats twenty words on Sunday, every time and it is not close. If you meet a word on Monday and again the following week and again a fortnight after that, you are done. That is the whole method.
Make it arrive rather than be fetched. Anything you have to remember to open, you will stop opening. This is the failure mode of every vocabulary app I have tried and of my own note full of words I never read again. A notification does not need you to remember anything, which is exactly its advantage over an app.
Use it once, badly. The fastest way to move a word from recognition into recall is to produce it yourself. Say it in a sentence out loud or drop it into a message to someone. It will feel forced the first time. That awkward first use is worth about five silent readings.
The bit nobody tells you
You are going to forget some of them anyway and that is completely fine. The goal was never a hundred percent retention. Meeting a good word regularly, at a moment when you have room for it, is a pleasant thing to do with thirty seconds of your morning even for the ones that do not stick. The ones that do stick are a bonus you get for free.
That is roughly the whole idea behind word, daily. One word, at a time you choose, pitched at a level you actually find interesting rather than one you already know. No list to work through, no streak to protect and nothing to open.
If you want to go further, the thing I would genuinely recommend is picking words from your own field rather than general vocabulary, because they turn up again on their own and that spacing happens without you doing anything. I have written about why a word from your own work sticks so much harder and if you want the practical setup there is a walkthrough of getting a word of the day notification onto your phone.