Any device that can be only programmed once, e.g. an EPROM or EPROM based processor without quartz window for erasure. Can be found rarely these days because EPROM has been mostly replaced by flash memory.
Generally a nonvolatile feature that can be only programmed once, either due to technological limitations or intentionally, e.g. for security reasons.
EPROM OTP , s. page 9. Until now, this is the most used OTP technology. Actually this is a electrically, in principle re-programmable non-volatile memory. For re-programming, these memory chips must first be erased by UV radiation, that's why real EPROMs (not OTPs) have ceramic packages with UV transparent windows. OTPs can be programmed (electrically) like EPROMs, but they are packaged in plastic packages (without such a window, of course) and so can't be erased. Plastic packages are a lot cheaper than ceramic packages with UV window.
There are even FLASH technology OTPs: FLASH memories actually are re-programmable, too, but these have a bug concerning their (electrical) erasing mechanism and so shouldn't be erased, hence they're marked and sold as OTPs.