noun

1. any of a large group of natural hydrous aluminum silicates of sodium, calcium, potassium, barium, etc., chiefly found in cavities in igneous rocks and characterized by a ready loss or gain of water of hydration: many are capable of ion exchange with solutions
2. a similar natural or synthetic silicate, used for softening water

Etymology: Swed zeolit < Gr zein, to boil (see yeast): from its swelling up when heated