acid
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Bite into a lemon and your face puckers. That sharp, sour sting on your tongue is the everyday calling card of an acid. Vinegar, citrus juice, fizzy soda, and the contents of your own stomach all share it. Acids feel sour because of something they release into water.
Chemically, an acid is a substance that donates hydrogen ions (H+, a bare proton) when it dissolves in water — or, more generally, any species that can hand off a proton to something else. The more readily it lets go of that proton, the stronger the acid. In water these freed H+ ions don't float alone; they latch onto water molecules to form hydronium (H3O+), and it is this excess of hydronium that makes a solution acidic.
This matters because that single donated proton drives an enormous range of chemistry: it dissolves metals and minerals, speeds up reactions, sets the pH of your blood, and etches, cleans, and digests. Be careful with the word strong, though: a strong acid is one that donates its proton completely, which is not the same as a concentrated or dangerous one — a dilute strong acid can be milder than a concentrated weak one.
Hydrochloric acid (HCl) in your stomach hands its proton entirely to water, making gastric juice strongly acidic so it can break down food and kill germs.
An acid is a proton donor; in water its protons form hydronium ions.
Strong and concentrated are different ideas. Strong describes how completely an acid releases its proton; concentrated describes how much acid is dissolved per litre. A bottle can be a dilute strong acid or a concentrated weak acid.