Source : Free On-Line Dictionary of Computing

One Time Programmable Read-Only Memory
     
         (OTPROM, EPROM OTP) A kind of storage device like an
        {EPROM} but with no quartz glass window in the package for
        erasing the contents.  This reduces the packaging cost but
        means the device cannot be erased with UV and so can only be
        written once.  Erasure is possible, but expensive, with
        X-rays.
     
        (1995-04-22)