The exact meaning of retention depends upon its context. For example:
a student's retention — what a student remembers (holds onto in memory)
freshman retention — the number of freshman who return to the school the next year
employee retention — how successful a firm is at keeping employees over the long term
water retention — the holding of water in the body — as in "a salt-free diet to reduce water retention"
retention bonus — a cash bonus paid to encourage a good employee to stay with a firm
retention basin — a ponding area — typically intended to hold stormwater
Fifteen years ago, the government had set aside all this money to clean up the White River by building more sewage retention pools and expanding this tunnel system that runs underneath downtown, diverting a creek called Pogue's Run.
There are no more uses of "retention" in Turtles All the Way Down.