The lozenge ◊ is a rhomboid shape usually referred to as a diamond. The name comes from the french word "losange", which means rhombus. It has four sides. Two obtuse angles and two acute angles usually measure 45°. It's usually used in decoration like wood mosaic and also in the decoration of ceramics, textiles, and silverware. It was also used on armor decoration and playing cards. In modal logic the lozenge gives the meaning of possibility. In programming languages it is used as a statement separator.
In axiomatic set theory, the lozenge denotes the principles known generally as the diamond principle.