What is the difference between aggregation and composition?

Several definitions exist for aggregation and composition based on the following elements:

Accessibility: The part objects are only accessible through the whole object. Lifetime: the part objects are destroyed when the whole object is destroyed. Partitioning: the whole object is completely partitioned by part objects; it does not contain any state of its own. Both aggregation and composition represent a whole-part relationship.

Definition [aggregation] aggregation is a relationship between part and whole in which: Part may be independent of the whole; the relationship between part and whole is long-term; parts may be shared between two containers. Aggregation is identified by the phrase is-part-of. Aggregation cannot be circular, i.e. an object cannot be part of itself.