All 3 Uses of

retention
in
Unbroken, by Hillenbrand

Dignity is as essential to human life as water, food, and oxygen. The stubborn retention of it, even in the face of extreme physical hardship, can hold a man's soul in his body long past the point at which the body should have surrendered it.
p. 183.4 *

retention = keeping

Louie ate voraciously, got stronger, and expanded exponentially, his face and body bloating from water retention.
p. 317.2

retention = being held in the body

But thanks to dramatic water retention, it was a doughy, moon-faced, muscleless weight.
p. 323.1

Definition:

to keep, hold, or remember something

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