One word, four different quantities
Someone who says their tea is not strong enough might mean that there is too little dissolved material in the water, that too little of what the leaf held has come out of it, that the cup does not grip the mouth enough, or that it does not contain as much caffeine as they were expecting. These are four separate things. They can be changed independently of one another, they respond to different adjustments, and two of them are properties of the liquid while the other two are properties of the experience of drinking it. Advice that does not distinguish them produces the very familiar situation in which a reader follows an instruction to make their tea stronger, gets a harsher cup that is no more intense than before, and concludes that the tea itself is at fault. The confusion is not pedantic: it sends people to the wrong lever, repeatedly, and there is no amount of care with the wrong lever that fixes the problem.
Concentration: how much is in the water
This is the sense the word usually ought to carry — the quantity of dissolved material per unit volume of liquid. It is set primarily by the leaf-to-water ratio, which caps what is possible, and secondarily by how much of the available material was actually collected during the steep. It is what changes when you add leaf, and it is what a strength setting on a machine is nominally adjusting. The important property of concentration is that raising it by adding leaf keeps the balance of what is dissolved roughly where it already was, since everything scales together: you collect more of each fraction in the same proportions. That is why more leaf produces a bigger version of the same cup rather than a different cup, and it is why it is nearly always the right first adjustment when a tea is disappointing.
Extraction: how much came out of the leaf
A quite different quantity: the proportion of what the leaf held that has actually been dissolved out of it. A high-ratio flash steep can be highly concentrated and lightly extracted — a great deal in the water, most of it still in the leaf — which is precisely why the leaf goes on giving for another ten infusions afterwards. A low-ratio long steep can be thoroughly extracted and still weak, with almost nothing left in the leaf and not much in the water either, which is why it cannot be re-steeped. These two cases feel utterly different in the mouth, and having a single word for both is worse than having no word at all. Raising extraction pushes further along the sequence of what leaves the leaf, so unlike raising concentration it changes the balance rather than the size, and it always brings the later fraction with it.
Astringency and bitterness, which are sensations
Astringency is a tactile drying sensation across the mouth and bitterness is a taste, and neither is a measure of how much material is in the cup. Both rise with extraction considerably faster than concentration does, which is why a long steep of a small quantity of leaf reads as strong while being genuinely thin — the drying grip is simply the loudest thing present, and loudness is easily mistaken for intensity. A great deal of what is described as strong tea in Britain is exactly this: heavily extracted, moderately concentrated, and carrying the astringency that milk was adopted to soften. Someone asking for a stronger cup and being handed a longer steep is being answered on this axis, whether or not it was the axis they meant, and the mismatch is invisible to both parties because both used the same word.
Caffeine, which people mean and rarely measure
The caffeine content of the liquor is a compositional fact about what was extracted, and it tracks the other three only loosely. Caffeine is highly water-soluble and comes out early and fast, so a short steep collects a large share of what is available while the liquor is still pale — meaning a light-looking early infusion may carry more of it than a dark late one from the same leaf. Leaf quantity matters far more here than steep length does, and water temperature matters more than either. Anyone using colour or perceived strength as a proxy is reading an unreliable indicator. TeaHQ does not publish caffeine figures for individual teas, because the variation between leaf, cultivar, harvest, grade and preparation is larger than most of the differences that get claimed between categories, and a single number would imply a precision that does not exist.
Why the confusion produces bad advice
The standard instruction to make tea stronger by steeping it longer raises extraction and astringency while barely raising concentration, which is exactly why it so often produces a cup that is harsher without being any more satisfying — and why the person who followed it concludes that they do not like strong tea after all. The instruction that actually works, in nearly every case, is more leaf for the same time. That lifts the concentration in the cup and leaves the balance alone. The reason the wrong advice persists so stubbornly is not ignorance but economics: extending a steep is free, and adding leaf costs money, so the free answer is the one that gets printed on packets and repeated. Being able to name which of the four you want makes the correct lever obvious immediately, and it is the most useful vocabulary in domestic brewing.
Saying what you mean
Thick, intense or full for concentration. Well-extracted or over-extracted for how far along the sequence the steep has gone. Astringent, drying or bitter for the sensations, keeping the first two apart from the third since they are different mechanisms. And caffeine content for caffeine content, stated as what it is rather than smuggled in under the word strength. This is not pedantry for its own sake: every one of these words points at a different adjustment, and a person who can name which one they are missing can fix their tea in a single move instead of three. It also makes it possible to ask a seller a question that has an answer, and to read a tasting note without having to guess which of four things the writer meant.