What is the definition of dehydration?

Answer 1

The basic definition of dehydrtion sythesis is a chemical reaction that builds up molecules by losing water molecules.

Answer 2

Hydrate means to give/add water. By adding the prefix de- an opposite word is made.

Dehydrate means to remove water. We often use it like this - I am dehydrated I need a drink. In this sentence the meaning is slightly different it means I am lacking water or my body has lost a lot of water.