"Post mortem" as a noun in English is short for "post mortem examination." It does refer to examining a dead body, but it's a common term in software development as well. It refers to the process of going over a completed project and figuring out what went right and what went wrong.

Jeff Atwood wrote an essay on it a while back if you want to read more about it: http://www.codinghorror.com/blog/2006/11/the-project-postmor...