lighten

American Heritage® Dictionary of the English Language, Fourth Edition

1. v. To make light or lighter; illuminate or brighten.
2. v. To make (a color) lighter.
3. v. Archaic To enlighten.
4. v. To become lighter; brighten.
5. v. To be luminous; shine.
6. v. To give off flashes of lightning.
7. v. To make less heavy.
8. v. To lessen the oppressiveness, trouble, or severity of. See Synonyms at relieve.
9. v. To relieve of cares or worries; gladden.
10. v. To become less in weight.
11. v. To become less oppressive, troublesome, or severe.
12. v. To become cheerful.
13. lighten up Informal To take matters less seriously: Everything will work out fine, so stop worrying and lighten up.

Century Dictionary and Cyclopedia

1. To become light or lighter; grow light or clear up; brighten: as, the sky lightens.
2. To emit flashes of lightning; shoot out as lightning; flash. See lightning. The lightning that lighteneth out of the one part under heaven shineth unto the other part.
3. To make light or bright; give light to; light up.
4. To illuminate mentally or spiritually; enlighten.
5. To send forth like lightning.
6. To become light or less heavy.
7. To make light or less heavy; reduce in weight; relieve of weight: as, to lighten coin by clipping or abrasion; to lighten a load or a ship.
8. To make less burdensome or oppressive; alleviate: as, to lighten the cares of life.
9. To cheer; gladden.
10. To make lighter in color or shade: as, to lighten the background of a picture.
11. To descend; settle down; light.

Wiktionary

1. v. transitive To alleviate; to reduce the burden of.
2. v. transitive To make light or lighter in weight.
3. v. transitive To make less serious or more cheerful.
4. v. transitive To make brighter or clearer; to illuminate.
5. v. intransitive To become light or lighter in weight.
6. v. intransitive To become less serious or more cheerful.
7. v. intransitive To become brighter or clearer; to brighten.

GNU Webster's 1913

1. v. To descend; to light.
2. v. To burst forth or dart, as lightning; to shine with, or like, lightning; to display a flash or flashes of lightning; to flash.
3. v. To grow lighter; to become less dark or lowering; to brighten; to clear, as the sky.
4. v. In this sense less common than light. To make light or clear; to light; to illuminate
5. v. In this sense less common than enlighten. To illuminate with knowledge; to enlighten.
6. v. To emit or disclose in, or as in, lightning; to flash out, like lightning.
7. v. To free from trouble and fill with joy.
8. v. To make lighter, or less heavy; to reduce in weight; to relieve of part of a load or burden
9. v. To make less burdensome or afflictive; to alleviate.
10. v. To cheer; to exhilarate.

WordNet 3.0

1. v. make lighter or brighter
2. v. reduce the weight on; make lighter
3. v. make more cheerful
4. v. become lighter
5. v. become more cheerful
6. v. alleviate or remove (pressure or stress) or make less oppressive