Latin for "Remember that you must die." The skull on the poet's desk is the most well-known memento mori, though in medieval and Renaissance times it was common to wear jewelry with a skull and bones or some other reminder of mortality. For example, Mary Queen of Scots had a pendant watch shaped like a coffin.
When Caesar was at the height of his popularity and power, the legend is that someone whispered to him "Remember, Caesar, thou art mortal."
The memento mori is not only a reminder that God will someday judge you. It is a warning against hubris. The Elizabethan fashion to keep a skull on your writing desk served as a reminder of the futility of human endeavors, of life's brevity and uncertainty. Remember Hamlet's "Alas, poor Yorick." speech.
"I make as good use of it (Bardolph’s face) as many a man doth of a death’s head or a memento mori."
"Do not speak like a death’s-head, do not bid me remember mine end."
- both quotes from Shakespeare
, Henry IV
As tribbel puts it, carpe diem was something of a replacement for memento mori.