To heat and slowly cool glass or metal to remove internal stress and make it stronger and less brittle.
From Old English 'onælan' (to set on fire), from 'on' (on) and 'ælan' (to burn); the process involves heat, so the etymology directly reflects the technique.
Annealing is why ancient glass artifacts survived thousands of years while freshly made glass without annealing shatters from tiny temperature changes—heat treatment is basically time travel prevention for materials!
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