3.11 Boxes
A box is like a single-element vector. It can print as a quoted #& followed by the printed form of the boxed value. A #& form can also be used as an expression, but since the resulting box is constant, it has practically no use.
Examples:
> (define b (box "apple"))
> b '#&"apple"
> (unbox b) "apple"
> (set-box! b '(banana boat))
> b '#&(banana boat)
Boxes in The Racket Reference provides more on boxes and box procedures.