A glass bong bowl with a 18mm joint size is an accessory designed to fit onto a 14mm female jointed bong. It is typically made of heat-resistant borosilicate glass, which allows for high-temperature use without the risk of cracking or shattering.

The bowl serves as the chamber where the dry herb is loaded for smoking. It typically features a narrow opening where the herb is packed, and a larger hole on the opposite side where the smoke is drawn through. The bowl may also feature a built-in screen or have a removable one to prevent herb from entering the water chamber and to improve airflow.

Glass bong bowls come in a variety of designs and colors, and some may include additional features like handles or carb holes. It is important to choose a bowl that fits properly onto the bong's female joint to ensure a secure and efficient smoking experience.

Overall, a 18mm glass bong bowl is an essential accessory for any smoker who wants a smooth and satisfying smoking experience.