Nullity of marriage or Nullity of domestic partnership.
Annulment implies that we are going to act as if a marriage did not take place; there are different bases on which someone can seek to annul a marriage.
Age
You can have a Marriage or a Domestic Partnership annulled if at the time the parties got into a Marriage or Domestic Partnership the one seeking to annul was under 18 years.
Prior Existing Marriage
Under the law you cannot be married to, or be in a domestic partnership with, more than one person. For example say you get married to person number 2 knowing you are married to person number 1 and your marital status has not been terminated with respect person number 1. Person number 2 can seek to annul their marriage for the obvious reason that it was not possible under the law to be married.
Unsound Mind
If someone enters into a marriage and at the time of entering into the marriage they had an unsound mind they could seek an annulment because at that time they would not have been able to understand the nature of the relationship they were getting into.
Force
If someone has been forced into a marriage, the relationship was not consensual, ergo they may be able to ask for an annulment.
Physical Incapacity
Someone may enter into a marriage under the understanding that their spouse can have children and they never consummated their relationship because the spouse is unable. Under this scenario they may have ground for an annulment