cadr

Car of the cdr in Lisp. The second element in a list.

If we let x denote the tree (1 2 3), then the car of x is 1, the cdr of x is (2 3), the cadr of x is 2, the caddr of x is 3, and the cdddr of x is nil.

Initialism of Clean air delivery rate, a measure of cleaned air from air purifiers.