A bell jar, also known as a cloche or display dome, is a bell-shaped vessel made of glass (or sometimes plastic) designed to cover objects or to contain gases or a vacuum.
In a household setting, they are normally used as a protective dust cover for the object covered by it. In a lab setting, they are called a bell jar and they are used to trap gasses or vacuums. In an agricultural setting, they are called a cloche and they are used to cover outdoor plants to protect them from frost.
A bell jar may or may not have a knob at the top to serve as a handle when picking it up.