lighten

1 lighten /ˈlaɪtn̩/ verb
lightens; lightened; lightening
1 lighten
/ˈlaɪtn̩/
verb
lightens; lightened; lightening
Britannica Dictionary definition of LIGHTEN
1 
[+ object] : to make (something) bright, light, or clear
2 
[no object] : to become bright, light, or clear

— lightener

/ˈlaɪtn̩ɚ/ noun, plural lighteners [count]
2 lighten /ˈlaɪtn̩/ verb
lightens; lightened; lightening
2 lighten
/ˈlaɪtn̩/
verb
lightens; lightened; lightening
Britannica Dictionary definition of LIGHTEN
1 
a  [+ object] : to make (something) less heavy, difficult, or severe
b  [no object] : to become less heavy, difficult, or severe
2 
a  [+ object] : to make (something) less sad or serious
b  [no object] : to become less sad or serious
3 
[no object] : to become less forceful

lighten up

[phrasal verb] informal
: to become more relaxed and informal : to stop being serious, worried, etc. — often used as a command