The Spanish term for the fermented agave juice distilled to make tequila is mosto.
In tequila production, harvested blue agave hearts are cooked and crushed to release sugary juice. That juice, together with soluble agave material, is fermented by yeast before distillation. The resulting fermenting or fermented liquid is commonly called mosto, a Spanish word meaning “must” or unfermented grape or fruit juice in broader beverage terminology.
Mosto should not be confused with pulque. Pulque is a separate traditional Mexican alcoholic drink made by fermenting aguamiel, the sap of mature agave plants; it is not the liquid distilled into tequila. Aguamiel also refers to the sweet sap itself, rather than the tequila-production ferment.
After fermentation, tequila producers distill the mosto, usually twice, to concentrate alcohol and create the spirit. The distillate may then be bottled as blanco tequila or matured to produce reposado, añejo, or extra añejo styles.