Mirepoix is a French word for the combination of three aromatics in cooking; carrots, celery and onions. Used most commonly in soups and stews, the three vegetables are sauteed in butter until tender. The most common proportion is 2 parts of onion:1 part of carrot:1 part of celery.
The Italian version of mirepoix is called soffritto and uses olive oil as the fat instead of butter. The aromatics include onion, carrot and celery, but in some cases the onion is exchanged for garlic and a little white wine may be included with the olive oil.