The story
From the children's story Goldilocks, in which one of the bowls of porridge the protagonist tries is neither too hot nor too cold, but just right.
Etymology from Wiktionary, CC BY-SA 4.0.
Goldilocks planet
Today it means: A planet which orbits in the habitable zone around a star, where temperatures like those on Earth occur, allowing for the possible existence of liquid water and of life. Full definition →