Optics

La lumière parasite is « stray light », and the word doing the work in it is one French uses as an adjective where English has only a noun. In English a « parasite » is a living thing feeding on another, and « parasitic » stays close to that; the unwanted signal creeping into an instrument has to be named afresh each time it turns up — « stray light » here, « background noise » in a recording, « spurious voltage » in a circuit, « interference » on the radio. French took the same noun and made a general-purpose adjective of it, meaning intruding, unwanted, coming from where it has no business: la lumière parasite, le bruit parasite, une tension parasite, and les eaux parasites that leak into a sewer. It went further and built a verb, parasiter, so a signal can be parasité — swamped by something that was never meant to be there. « To parasitise » in English does only what a tapeworm does. The bridge is in the oldest sense of the word: the Greek parasitos was the man who ate at somebody else’s table, and uninvited is exactly the idea French kept. So next to a technical noun, parasite never points to an organism.