Widow's blood

"Widow's blood forces a man's bowels and bladder to fail. He drowns in his own poisons; a slow and painful death. It has earned it's name many times, I am sure."

- Oberyn Martell

Widow's blood is a poison. It causes the victim's bowels and bladder to fail and they die a slow and painful death, drowning in their own poisons.

In the books
In the A Song of Ice and Fire novels, Widow's blood is named so for its color. It shuts down the bladder and bowels until the person dies from the body's own poisons.

During Tyrion Lannister's trial for the presumed murder of King Joffrey Baratheon, Grand Maester Pycelle testifies that Tyrion had stolen several substances of his, including Widow's blood, although Pycelle confirms that Joffrey's death was caused by the strangler.