Ichor

Ichor is the golden blood of the gods and Titans (immortals bleed ichor instead of blood).In battle they can be wounded but never killed, partly because they have Ichor. Demigods also have their godly parent's ichor in their veins infused with their human blood, however it cannot be seen.

In Greek mythology, Ichor is the ethereal fluid that is the Greek gods' blood, sometimes said to have been present in ambrosia or nectar. When a god was injured and bled, the ichor made his or her blood poisonous to mortals. Great demigods and heroes occasionally attacked gods and released ichor, but gods rarely did so to each other in Homeric myth. Ichor was considered to be golden in color. Although Demigods have Ichor in their human blood stream, it does not make them entirely immortal. The ichor fused with human blood may be the cause of being a half-blood.

Apart from mythology, poets since Victorian times have used it to mean a divine drink, often wine.

The Lightning Thief
Percy stabs Ares in his ankle and makes him bleed Ichor, which dissapated into the tide (they were fighting in water).

The Titan's Curse
Percy has a dream when Artemis was captured and she was bleeding Ichor while captured by Atlas in chains.

The Last Olympian
Typhon bleeds Ichor when attacked by Poseidon. It made a very tall waterfall (temporarily).

The Lost Hero
Jason hits a Venti with a club, and he bled Ichor.