What are the defining traits of a memoir?

A memior is autobiographical by nature, but is typically about a particular time or event in a person's life. For example,

A woman wrote about her first husband's death. This had happened over 20 years ago. He was helping a neighbor he hardly knew load up his vehicle. The truck moved unexpectedly, and he was thrown to the ground and sustained a head injury so severe that when they got him to the emergency room he was declared brain-dead. Hours later she was standing on the roof of the hospital with her husband's brother, deciding whether or not to take him off life support. She was wearing flip-flops, shorts, and a T-shirt, and she remembers thinking how these were the wrong clothes to be wearing at such a moment....

So, essentially, the woman's memoir was about a profound event in her life, not her entire life (as characteristic of an autobiography).