Which memory is non volatile and may be written only once?

A programmable read-only memory (PROM) or field programmable read-only memory (FPROM) or one-time programmable non-volatile memory (OTP NVM) is a form of digital memory where the setting of each bit is locked by a fuse or antifuse. It is one type of ROM (read-only memory). The data in them is permanent and cannot be changed.

PROMs are used in digital electronic devices to store permanent data, usually low level programs such as firmware (microcode).