Word for the Wise June 08, 2007 Broadcast Topic: Retention and detention
A question about retention versus detention held our curiosity. Since retention refers to an act of retaining and detention to an act of detaining, we'll keep our curiosity moving in the direction of the verbs behind the nouns. (来源：英语学习门户网站EnglishCN.com)
Retain is rich in meanings: it can mean "to keep in possession or use; to keep in one's pay or service; to keep in mind or memory;" or "to hold secure or intact." Detain, meanwhile, can mean "to hold or keep in or as if in custody;" or "to restrain, especially from proceeding."
Both retain and detain share an ancestor in the Latin tenere, meaning "to hold." The de in detain means" from; away;" the re in retain means "again; anew." It's easy enough to understand the sense development of words meaning "to hold away (from)" and "to hold again" or "anew," but hold on as we distinguish between the keeping associated with "retaining" and the keeping associated with "detaining."
Detain implies keeping something for longer than expected or usual, a delay in letting go from one's control; retain implies continued keeping, especially against a threatened taking or loss.
Where does keep fit in? Hold onto your hats: the Old English ancestor of that very old word meant "to observe, seek, heed," or "seize."