1 illuminate, brighten, light up:
Opening the curtains would lighten up the room.
2 cheer (up), brighten, gladden, shine; smile:
Her face lightened when he said he would be home for the holidays.
disencumber, disburden, relieve, alleviate, reduce, lessen, mitigate:
To lighten my load, I had left my suitcase at the hotel.