The primary ingredient in mustard is seeds from the mustard plant. These seeds come in a few varieties, including yellow, brown, and black, which influence the flavor and heat of the mustard. The seeds are usually ground into a powder and mixed with water, vinegar, salt, and sometimes other spices to create the condiment. Mustard is widely used in various cuisines around the world, appreciated for its tart and tangy flavor, making it a popular choice for dressings, marinades, and as a condiment for meats.