means "remember that you must die" in latin.
word from a very impressive roman tradition. during the roman republic, only war-winning generals were allowed to wear a crown with the name corona civica during the victory parade on the streets of rome*, the duty of the slave holding this crown on the general's head was not only to hold the crown, but also to keep the victorious general from getting into the air* constantly in the ear of the general. whispering the word "memento mori" (remember you are mortal).
the name of an episode of xfiles in season 4
from his point of view, he may also support carpe diem. like: "don't forget you will die anyway, enjoy your life"
it is one of the important principles of the stoics.. it means "remember death", it is also an expression used in baroque art.