A protective covering for the soil or compost, old leaves, paper, rocks, etc. – indoors, mulch keeps soil too moist and possible fungus could result from and outdoors mulch helps the soil retain and attract moisture.
Merriam-Webster Online Dictionary
mulch (noun) a protective covering (as of sawdust, compost, or paper) spread or left on the ground to reduce evaporation, maintain even soil temperature, prevent erosion, control weeds, enrich the soil, or keep fruit (as strawberries) clean
mulch, cover crop, vegetation cover, humus, decayed organic matter, fertilizer, surface application, soil amendment, nutrient layer, soil amendment, soil improver, soil enhancer, mulch, organic cover, protective layer