Also: reduce, reduction, simmer down
Reducing
Reducing is simmering a liquid hard enough to evaporate water, concentrating whatever is dissolved in it.
Why it matters. Flavour compounds stay behind while water leaves. Sauces gain body, glazes gain stick, stocks intensify. A reduction also deepens colour — the sauce darkens as it tightens.
How to use it. High heat, wide pan, no lid. Watch for the change in surface — small rolling bubbles mean it's still thin, slow glossy bubbles mean it's coming together. A spoon drawn across the pan should leave a track that closes slowly when the sauce is ready.
Examples
(awaiting examples from cooks)