Scientists use the word lactic for substances that have to do with milk. Lactic acid is the substance that makes things like yogurt and buttermilk taste sour.
The term lactic comes from the French lactique and its Latin root, lac, or "milk." The word is almost always connected to acid — lactic acid, also known as lactate, is the byproduct of a chemical reaction that occurs in milk as it ferments. Lactic acid is also present in our bodies, as it's produced by muscles that are working hard, in our guts, and even in our mouths.