What is the definition of an acid in chemistry? What is an acid according to chemistry? The term 'acid' has its origin in the Latin word acidus, meaning sour. In fact, anything that tastes sour contains an acid. For example, lemon juice, tomato, vinegar, etc., all taste sour. So, each of these substances must contain an acid.Substances containing an acid (from Latin acere … [Read more...] about What is the definition of an acid in chemistry