Trivial changes to Kaysie's def., I'd declare this done:
Substrate - The material at the bottom of the tank. Most terrestrial tanks have a substrate of damp paper towels, topsoil, leaf litter, coconut fiber, or a mixture of different materials. Aquatic tanks may have a substrate consisting of sand, gravel, river rocks, slate, or some other non-soluble material. A substrate should be chosen for the needs of the species and the keeper: appropriate size, natural appearance, compatibility with live foods, ease of cleaning, etc.