A more thorough explanation:

Definition: A spouse is a person's husband or wife through legal marriage.

Examples:

• Innocent spouse: This refers to a spouse who can be relieved of tax liability for income that the other spouse did not report on a joint tax return. For example, if one spouse failed to report income without the knowledge of the other spouse, the innocent spouse may be able to avoid being held responsible for the taxes owed.
• Putative spouse: This refers to a spouse who believes in good faith that their marriage is legally valid, even if it is later found to be invalid. For example, if a couple gets married but later discovers that their marriage license was not valid, one of the spouses may still be considered a putative spouse if they believed in good faith that their marriage was legal.
• Surviving spouse: This refers to a spouse who outlives their partner. For example, if a husband passes away, his wife would be considered his surviving spouse.

These examples illustrate the definition of a spouse as someone who is legally married to another person. The innocent spouse example shows how a spouse can be relieved of tax liability if they were not aware of their partner's actions. The putative spouse example shows how someone can still be considered a spouse even if their marriage is later found to be invalid. The surviving spouse example shows how a spouse can be referred to after their partner has passed away.