Longman Dictionary of Contemporary English
Wiktionary
n. any of various forms of processed yeast products used as food additives or flavourings.
Wikipedia
Yeast extract is the common name for various forms of processed yeast products made by extracting the cell contents (removing the cell walls); they are used as food additives or flavourings, or as nutrients for bacterial culture media. They are often used to create savoury flavours and umami taste sensations, and can be found in a large variety of packaged food including frozen meals, crackers, snack foods, gravy, stock and more. Yeast extracts in liquid form can be dried to a light paste or a dry powder.
Yeast extracts and fermented foods contain glutamic acid, an amino acid that’s found in meat, cheese, and vegetables including mushrooms, broccoli, and tomatoes. According to Joe Dickson, a past member of the USDA’s National Organic Standards Board,
Usage examples of "yeast extract".
Found in liver, heart, red meat, beans, asparagus, molasses, oatmeal, soya beans, yeast extract, almonds.
But you also get tyramine in yeast extract, brewer's yeast, and meat extracts such as Marmite or Oxo.