In Greek mythology, Pandora opened a jar, releasing troubles into the world. The ancient Greek source describes the container as a pithos, a large storage vessel normally used for commodities such as grain, wine, or oil. It was not originally called a box.
The familiar phrase “Pandora’s box” arose later through translation history. The sixteenth-century humanist Erasmus rendered the Greek vessel with the Latin word pyxis, meaning a small box or casket. That choice helped establish the expression used in many modern languages.
The jar’s contents are described as harmful forces or evils that had previously been kept away from human life. After Pandora replaced the lid, one thing remained inside: hope. The story is therefore about the sudden arrival of suffering, labor, illness, and other hardships, while also preserving a disputed but important symbol of hope.
A pithos was considerably larger than an ordinary household jar, making “container” a useful modern description, but “jar” is the standard concise answer.