A greenhouse gas is a gas that traps heat in the Earth’s atmosphere and contributes to the greenhouse effect [the process by which heat is trapped in the Earth’s atmosphere]. Examples of greenhouse gases include carbon dioxide, methane [a gas produced by natural processes and human activities such as agriculture and waste management], and water vapor.
Very Simple Definition:
A greenhouse gas is a gas that makes the Earth warmer. It is like a blanket around the Earth that keeps it warm. Some examples of greenhouse gases are carbon dioxide, methane, and water vapor.