"Zeolite" refers to a group of minerals that are basically hydrated calcium potassium sodium aluminosilicates in which the water is held in cavities in the lattice.
The lattices are negatively charged and they loosely hold cations such as calcium, sodium, ammonium, and potassium; and also water. Their ability to exchange one cation for another is known as their "cation-exchange capacity" or "CEC". Cation-exchange capacity is a measure of the number of cations per unit weight available for exchange, usually expressed as milliequivalents per 100 grams of material.