In general medical terms post op means post - operative or after an operation/surgery.
Specifically in sexual terms in refers to a transexual who has had the operation to change gender usually from man to woman, but can be woman to man too.
So in most cases a post -op transexual will refer to a man who has had a sex change operation and his now a woman.
Basically the cock and balls come off and usually a vagina is constructed, plus boob job, hormone therapy etc.
A transexual who has changed from male to female but still has the cock and ball is NOT post-op.
Some post ops are very convincing and hard to tell from a natural woman, especially the Asian ones.
Even some trannies that still have the cock and balls are not easy to identify until you get them naked and then you get a big surprise. LOL!
With the post-op ones even when you get them naked you have to have a really close look and feel before you can be sure.