Dependency happens when you can't function without the help of someone or something. If you have a dependency on coffee, you need it to be human in the morning. Not sure? Ask the people you live with.
A dependency on the help or support of another person isn't necessarily negative, but other kinds of dependency — on something habit-forming or addictive, like cigarettes — can be harmful. You can also use this noun to mean "a country, province, or territory controlled by a bigger, more powerful country." Guam and Puerto Rico, which don't have complete independence from the US, can be called dependencies.