Cons

(cons argument1 argument2) -> pair

Creates a pair, composed of the first argument or car and the second argument or cdr. To distinguish the result from a list, it is displayed as
(argument1 . argument2).

In fact, all lists in Scheme, which this portion of Mosquito Lisp is derived from, have their roots in (cons).
Example:

>(cons 'a 1)
:: (a . 1)