to reduce the amount of work, worry, debt etc that someone has [≠ increase]
lighten the load/burden/workload
We should hire another secretary to lighten Barbara's workload.
2 [intransitive and transitive]
to become brighter or less dark, or to make something brighter etc [≠ darken]:
As the sky lightened, we were able to see where we were.
3 [intransitive and transitive]
if you lighten something such as a mood or atmosphere, or if it lightens, it becomes less sad or serious
lighten the atmosphere/mood/conversation
Nora didn't respond to my attempts to lighten the conversation.
if someone's face or expression lightens, they begin to look more cheerful:
His whole face would lighten when anyone mentioned Nancy.
5 [intransitive and transitive]
to reduce the weight of something or become less heavy
used to tell someone not to be so serious about something:
You need to lighten up a bit.