Pollution is the presence or introduction of harmful substances or pollutants [substances that make air, water, soil etc dangerously dirty] into the environment (such as air, water, and soil), which can cause damage or discomfort to living organisms. It can also refer to the act of contaminating the environment. For example, when factories release smoke into the air or when oil spills into the ocean, it causes pollution.
Very Simple Definition:
Pollution is when harmful things get into the air, water, or ground, and can make living things sick or hurt the environment. For example, when smoke comes out of factories or when garbage is thrown into the river, it causes pollution.