mourn ‧ ing /ˈmɔːnɪŋ $ ˈmɔːr-/ BrE AmE noun [uncountable]
1 . great sadness because someone has died:
The Armenian authorities declared May 29 a national day of mourning.
in mourning (=feeling great sadness)
It was the custom to visit those in mourning and sit quietly with them.
2 . black clothes worn to show that you are very sad that someone has died:
She was recently widowed and wearing mourning.