Glass block, also known as a glass brick.
Here's some information about glass blocks:
Purpose: They are architectural elements made of glass that allow light to pass through while providing visual privacy due to their semi-transparent nature.
Appearance: They can be clear or colored, and can have textured or smooth surfaces. The block in the image has a textured surface, which helps diffuse light.
Construction: They are typically made by joining two hollow halves under heat and pressure, creating a hollow center that provides some insulation.
Applications: Glass blocks are used in various settings, including bathrooms for privacy, exterior walls for natural light and security, and interior partitions to create a light and airy feel. They can also be used in horizontal applications like ceilings and walkways.