Sweating: The gentle heating of something in little oil or butter over very low heat without browning, resulting in tender and often translucent pieces.
When sweating, the pan being used is usually covered so the lid traps steam, which will condense and drip back into the pan. The purpose of sweating is to soften and release moisture from the food (usually aromatic vegetables and alliums: onions, shallots, garlic, celery) in the beginning stages of a dish, usually before adding them to a stew or other plate. The release of moisture is how the term “sweating” gets its name.
When to Sweat: Perspire those vegetables before adding to soups, stews, risottos or sauces.
Sweating Is Not the Same As Sautéing: A higher heat is used in sautéing, often resulting in browning. Sweating is about the softening, sautéing is for browning.