A copper-containing protein found in the blood of many invertebrates like crustaceans and mollusks that carries oxygen, similar to hemoglobin in vertebrates.
From Greek 'haima' (blood) + 'kyanis' (dark blue). Named in the 19th century because solutions of this protein appear blue due to the copper atoms it contains, unlike the red color of hemoglobin.
While your blood is red because of iron-based hemoglobin, octopuses and horseshoe crabs have blue blood powered by copper-based hemocyanin—it's equally efficient but creates an alien aesthetic that's helped scientists understand how completely different chemistry can solve the same biological problem.
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