Mustard is a plant, of the Brassica persuasion, whose small seeds are used as a spice, and are also mixed with liquids such as water or vinegar to make a condiment, also referred to as "mustard".
One variety of the plant grows throughout Europe and North Africa; a second originates in the Himalayas; a third is native to the Americas.
The seeds are about 2mm in size, coloured from yellowish white to black. It was the Ancient Romans who mixed crushed mustard seeds with grape must to make "fiery must" -- must-ard.
The strength (or fireiness) of mustard is dependent upon the ingredients used. Black seeded mustard is generally reckoned to be the hottest, and the use of temperature in the mixing has an inverse effect -- that is, the same mixture, when made with cold water will taste hotter than a hot-water mix, and cooking mustard also reduces its potency.