Tetracalcium phosphate (TTCP, CAS 1306-01-0) is a calcium phosphate ceramic with the chemical formula CaД(POД)ВO. It is a high-temperature phase, typically prepared by solid-state reaction at temperatures above 1300░C, and is one of the few calcium phosphates with a Ca/P molar ratio of 2.0. Structurally, it is closely related to hydroxyapatite but contains an oxide ion, making it highly reactive in aqueous environments. Its primary application is in biomedical materials, particularly as a key component in self-setting calcium phosphate cements (CPCs). When mixed with other calcium phosphates like dicalcium phosphate anhydrous (DCPA) or dicalcium phosphate dihydrate (DCPD), TTCP undergoes an isothermal setting reaction to form hydroxyapatite, the mineral phase of bone and teeth. This makes it invaluable for bone defect repair, dental restorations, and as a bone graft substitute. In this context, it functions as a reactive filler and a precursor for in-situ hydroxyapatite formation. Additionally, TTCP serves as a precursor for advanced bioceramics and functional coatings. Its high reactivity and biocompatibilitlity also allow it to be used as a raw material for synthesizing other calcium phosphate-based materials, including drug delivery systems and tissue engineering scaffolds. Due to its specific role in forming hard tissue, it is not a general synthetic building block and should not be classified under synthetic chemistry.