Dependency

n. pl. Dependencies ().

1. State of being dependent; dependence; state of being subordinate; subordination; concatenation; connection; reliance; trust.
2. A thing hanging down; a dependence.
3. That which is attached to something else as its consequence, subordinate, satellite, and the like.
4. A territory remote from the kingdom or state to which it belongs, but subject to its dominion; a colony; as, Great Britain has its dependencies in Asia, Africa, and America.

Dependence is more used in the abstract, and dependency in the concrete. The latter is usually restricted in meaning to 3 and 4.