The Röntgen equivalent physical or rep (symbol rep) of tissue.
At the time, this was thought to be the amount of energy deposited by 1 roentgen. Improved measurements have since found that one roentgen of air kerma deposits 8.77 mGy in dry air, or 9.6 mGy in soft tissue, but the rep was defined as a fixed number of ergs per unit gram.
A 1952 handbook from the US National Bureau of Standards affirms that "The numerical coefficient of the rep has been deliberately changed to 93, instead of the earlier 83, to agree with L. H. Grays energy-unit." Grays energy unit was " one roentgen of hard gamma resulted in about 93 ergs per gram energy absorption in water". The lower range value of 83.8 ergs was the value in air corresponding to wet tissue. The rep was commonly used until the 1960s, but was gradually displaced by the rad starting in 1954 and later the gray starting in 1977.