Simple Definition:

Alchemy is an ancient practice that combines spiritual and scientific beliefs in an attempt to transform materials into something of greater value, such as transforming lead [a type of metal] into gold. It involves the use of complex symbols to represent different elements and processes. Alchemists believed that by achieving a state of spiritual purity, they could unlock the secrets of the universe and achieve eternal life.

For example, alchemists would perform experiments with various substances, such as metals and minerals, in an effort to create the philosopher’s stone, a legendary substance that was believed to have the power to transmute [transform] base metals [common and inexpensive metal] into gold and grant immortality.

Very Simple Definition:

Alchemy is an old belief that some people had that they could change ordinary things like rocks into valuable things like gold. They used special symbols to represent different elements and ideas. Alchemists thought that if they were pure in their hearts and minds, they could learn the secrets of the universe and live forever.

For example, alchemists would try to turn rocks into gold by mixing them with other things and heating them up. They also believed that there was a special stone that could help them live forever and make ordinary things valuable.