A spice is essentially any part of a plant which is ground up and added to dishes of food to add flavor. Such plants typically have low levels of moisture to begin with and are usually dried to preserve their shelf lives. They are never eaten on their own.
Only comparatively small amounts of spices are added to dishes, as they have very pungent, distinctive flavors. This makes them quite distinct from other ingredients such as vegetables or grains which will have a much milder flavor and which will make up a substantial portion of a dish. Spices will not be apparent to the eye beyond a possible color change.