In Greek mythology, ichor is the blood of the gods. It is sometimes said to come from the ambrosia and nectar they consume, a feature of several Proto-Indo-European-descended religions, which they must continue to consume or else lose their immortality (or alternately their ability to function). Although in the case of the Greek gods specifically this was less than clear cut.
Ichor has gone on to have additional meanings beyond the Greek origin. In now archaic medical usage it refers to "watery discharge from a wound."
In the video game series Command & Conquer, "ichor" is the Scrin's name for Tiberium. You can probably guess why.
In some urban fantasy fiction, "ichor" is a term for the blood of vampires. This may be seen in Marvel's take on vampires and elsewhere online.
Sometimes authors who don't know any better use it as a synonym for blood, especially the blood of demons and Lovecraftian monsters. Author Ursula K. Le Guin, in her essay "From Elfland to Poughkeepsie," calls the term "the infallible touchstone of the seventh-rate."
And sometimes ichor is the urine of the gods. I think I'm gonna be sick...