memento mori [məˈmɛntoʊ ˈmɔraɪ] n.
1.) A reminder of death or mortality, especially a death's-head.
2.) A reminder of human failures or errors (American Heritage Dictionary of the English Language).
Etymology: New Latin memento mori, be mindful of dying : Latin memento, sing. imperative of meminisse, to remember + Latin mori, to die.
"Bardolph: Why, Sir John, my face does you no harm.
Falstaff: No, I'll be sworn; I make as good use of it as many
a man doth of a Death's-head or a memento mori: I
never see thy face but I think upon hell-fire and
Dives that lived in purple; for there he is in his
robes, burning, burning" (Henry IV Part One, William Shakespeare, 1597)