Mathematics. ( 1 of a topology or topological space) having the property that every subset is an open set. 2  defined only for an isolated set of points: a discrete variable. 3  using only arithmetic and algebra; not involving calculus: discrete methods.
limestone - a sedimentary rock consisting mainly of calcium that was deposited by the remains of marine animals. atomic number 20, Ca, calcium - a white metallic element that burns with a brilliant light; the fifth most abundant element in the earth's crust; an important component of most plants and animals.
Jupiterimages/Photos.com/Getty Images. Limestones are a class of sedimentary rocks composed of the same mineral, calcite, which is made from calcium carbonate, or CaCO3. Despite this similarity in composition, however, not all limestone rocks are the same. Chalk is actually a special form of limestone that exhibits some differences from other rocks in the same class.