Formula  n. (pl. -s or (esp. In senses 1, 2) -lae) 1 chemical symbols showing the constituents of a substance. 2 mathematical rule expressed in symbols. 3 a fixed form of esp. Ceremonial or polite words. B words used to formulate a treaty etc. 4 a list of ingredients. B us infant's food. 5 classification of a racing car, esp. By engine capacity.