The term FAT, SAY can be solved by the crossword solution LIPID, which is a five letter word. Lipids are a group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins, monoglycerides, diglycerides, triglycerides, and phospholipids. They are an important part of biological structures and serve as a source of energy for organisms. Lipids are also involved in many metabolic processes, including the synthesis of hormones and cell membranes.