Hi Robert,
The term "post mortem" refers to the process/meeting/workshop related to assessing the project's successes, failures etc. Its output should generally result into a lessons learnt document, which makes it part of the project closing phase.
It could be applied to both successful and early terminated projects, and its main focus is to retain what was done well, identify the mistakes and learn from the experience.
I do not think the term will come up in PMP exam, but as long as you know its intention, you should be able to understand where it fits.
Just as an aside, i like to use the post mortem term, and generally run the sessions along an agende of: what should we keep doing, what should we stop doing and what should we should start doing. This generally opens up the floodgates to result in an effective lessons learnt output.
Regards