n [U] black or dark clothes worn as a (conventional) sign of grief at sb's death 丧服（黑色或深色的）
*When grandmother died they went into (ie started to wear) mourning. 祖母逝世时他们开始服丧.
* She was in mourning for a month. 她服丧一个月.

n[U]
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.
black clothes worn to show that you are very sad that someone has died
　She was recently widowed and wearing mourning.