Retention

Retention can have the following meanings:

Instance of retaining (e.g. water in the ground; see retention pond or retention basin)
In learning: the ability to retain facts and figures in memory (see: memory retention, spaced repetition)
In usenet, the retention period is the time a news server holds a newsgroup posting before deleting it as no longer relevant. For practical reasons, most news servers have different categories for different classes of postings: for example, the "announce" newsgroup for the ISP or other agency providing the news server may retain postings either for a very long time or indefinitely, whereas postings in binary newsgroups are usually not retained for long due to the storage requirements involved.