Not just "something", but something that has "died" or ceases to exist in its prior form and the "postmortem" is generally the detailed analysis of what happened.

Also, http://www.merriam-webster.com/dictionary/postmortem

The second definition given does not refer to "death" at all and is commonly used in science and business contexts. It usually refers to the project (or the "event") being over at the point of discussion.

The term is overloaded nowadays, as can be seen on Wikipedia: http://en.wikipedia.org/wiki/Post-mortem_%28disambiguation%2...

Now it typically connotes a post-event review using the metaphor of the autopsy.

"retrospective" is better.