less ‧ en /ˈles ə n/ BrE AmE verb [intransitive and transitive]
to become smaller in size, importance, or value, or make something do this SYN reduce
lessen the risk/chance/possibility etc (of something)
Exercise lessens the risk of heart disease.
lessen the impact/effect/importance (of something)
The new project will lessen the effects of car pollution.
Gradually her anxiety lessened.