Liquid
Shopify's templating language for themes.
Liquid is the open-source templating language Shopify uses to build theme files. It lets a theme pull dynamic store data — products, collections, cart contents — into HTML using tags, objects, and filters. Fluency in Liquid is what separates a truly custom Shopify build from a lightly tweaked off-the-shelf theme, and it is core to the stores we develop.