Mourning is behaviour in which you show sadness about a person's death.
e.g. The period of mourning and bereavement may be long...
e.g. Expect to feel angry, depressed and confused. It's all part of the mourning process...
If you are in mourning, you are dressed or behaving in a particular way because someone you love or respect has died.
e.g. Yesterday the whole of Greece was in mourning...
e.g. The boys wore black tail coats in mourning for George III.