Simple Definition:

Hormones are chemicals used as messengers that are produced by glands [organs that make and release chemicals] in the body and travel through the bloodstream to regulate [control] various bodily functions [actions or processes within the body]. For example, the hormone insulin helps control blood sugar levels, while the hormone estrogen regulates the menstrual cycle [the monthly series of changes the body goes through to prepare for pregnancy] in females.

Very Simple Definition:

Hormones are special chemicals in our body that tell it how to work properly. They help us do things like grow, feel hungry, and control our emotions. For example, when we eat something sweet, our body releases a hormone called insulin that helps keep our blood sugar in balance.