Non Return to Zero
From Computer History Wiki
Non Return to Zero (usually given as the acronym, NRZ) is an encoding technique which is commonly used on magnetic storage and serial lines.
In NRZ, whether a particular bit is '0' or '1' is indicated by the signal level during each clock time; in other words, there is no 'rest' state between bits.
The disadvantage of NRZ is that a long strings of either value do not cause any level transitions (hence the name), so a separate clock is always needed, because it cannot be self-clocking (unless used with a periodic explicit clock, as in asynchronous serial lines; or with either bit stuffing or run-length limited coding, to limit the maximum number of transition-less bit times).
