Retain means to hold on to or keep. People who can retain a lot of information are often mistaken for geniuses, but really they just have very good memories.
To retain is to keep or maintain, whether in mind, possession or a certain condition. If you have a great personal assistant, you'll probably want to retain him for as long as you can. If your body is retaining water you might look a little plump. The verbs restrain and detain sound similar, but they mean to keep something by force.
