definition of lessen
LINK / CITE ADD TO WORD LIST. verb. The definition of lessen is to diminish or to cause something to be diminished or reduced. When you take pain pills and they reduce pain, this is an example of a situation where the pills lessen the pain.erb. The definition of lessen is to diminish or to cause something to be diminished or reduced. When you take pain pills and they reduce pain, this is an example of a situation where the pills lessen the pain.