In the Greek myth of Pandora, the container she opened was a pithos.
A pithos was a large storage jar used in ancient Greece for products such as grain, oil, and wine. The earliest surviving literary account, Hesiod’s Works and Days, describes Pandora opening a pithos rather than a small box. The familiar phrase “Pandora’s box” arose later through a translation tradition.
The word pithos refers to the vessel’s type, not to a unique mythological object. Large pithoi were often kept in houses or storerooms, making a jar a natural container in the story. The myth says that evils escaped from it, while hope remained inside.
The common mix-up comes from Erasmus of Rotterdam, whose Latin translation used the word pyxis, meaning a box or small container. That wording helped establish the expression used in English today.