Is it farther or further?
late 14c., from Latin retentionem (nominative retentio) "a retaining, a holding back," noun of action from past participle stem of retinere (see retain). Originally medical; mental sense is from late 15c.
retention re·ten·tion (rĭ-těn'shən)
Involuntary withholding by the body of wastes or secretions that are normally eliminated.
The holding by the body of what normally belongs in it, such as food in the stomach.
An ability to recall or recognize what has been learned or experienced; memory.
In dentistry, a period following orthodontic treatment when a patient wears an appliance or appliances to stabilize the teeth in their new position.