The dictionary definition of "marital status" is merely the condition of being married or unmarried. In fact, there are many different categories of marital status, and it is a question that is often asked on government forms.
One common category of marital status is "married." In the UK, marriages are legal unions between two people according to certain requirements, such as age.
Never been married
One category of single is "never been married." These are people who have not been bonded in matrimony.
This category includes people whose spouse has passed away. They were legally married, but now are considered to be widowed after the death of their spouse.
Divorced or separated
These are marriages that have ended, permanently or temporarily, for reasons other than death. A couple can be legally separated, or their marriage can be dissolved by a legal divorce decree.
There are other categories for marital status too, including being civil partners, which is not technically considered the same as being married.