The term 'Essential Substance in Food' can be summarized as a VITAMIN, which is a seven-letter word. Vitamins are organic compounds that are essential for the body to function properly and are found in food. They are necessary for growth, development, and overall health. Vitamins are divided into two categories: fat-soluble and water-soluble. Fat-soluble vitamins are stored in the body's fatty tissues and are not easily excreted, while water-soluble vitamins are not stored in the body and must be replenished regularly.