Merriam-Webster's Advanced Learner's Dictionary:
mourning
mourn·ing /ˈmoɚnɪŋ/ noun [noncount]
1 : the act of mourning for someone who has died
• a day of national mourning
• She is still in mourning for her dead husband.
• The whole town was in mourning.
• She went into mourning for her dead husband. [=she began to mourn for her dead husband]
2 : great sadness felt because someone has died
• a period of deep mourning
3 : black clothing that is worn to show that you are mourning for someone who has died
• His widow was dressed in mourning.