(1):

(a.) To make less; to reduce; to make smaller, or fewer; to diminish; to lower; to degrade; as, to lessen a kingdom, or a population; to lessen speed, rank, fortune.

(2):

(v. i.) To become less; to shrink; to contract; to decrease; to be diminished; as, the apparent magnitude of objects lessens as we recede from them; his care, or his wealth, lessened.