A postmortem is for reflecting on something that went so wrong you had to kill it, and you want to objectively describe what led to that, and what lessons can be learnt from the failure.
A retrospective is just the reflecting part, without implying you believe it is going to fail and you're looking for some schadenfreude ;)
A retrospective is just the reflecting part, without implying you believe it is going to fail and you're looking for some schadenfreude ;)