Dependence is a state where someone is heavily reliant on someone or something else. This could mean the dependence of children on parents or an addict on a drug.
Since to depend on someone is to need them, dependence is a state or condition of strong need. There are many kinds of dependence: a dog is in a state of dependence with his master. This word is also often used for drug and alcohol dependence, which is known as addiction. Dependence a strong word for extreme needs: liking something, like chocolate, isn't really dependence. The opposite of dependence is independence: being self-reliant and not needing others.