Simple Definition:

A virus is a type of tiny organism [a type of living thing] that can cause diseases in plants, animals, and humans. Viruses have genetic material [the instructions that tell an organism how to grow and function] and can reproduce [make copies of themselves] by invading and taking over the cells of a host organism [the animal or plant the virus invades]. Examples of viruses include the flu virus, the common cold virus, and the coronavirus.

Very Simple Definition:

A virus is a tiny living thing that can make people and animals sick. It is so small that we cannot see it without a special tool called a microscope. Examples of viruses are the germs that cause flu and the cold.