An isorem was a measurement of radiation.

In 2376, the Vaadwaur homeworld was heavily contaminated with radiogenic particles at around 6,000 isorems at the highest point. (VOY: "Dragon's Teeth")

In 2378, antimatter radiation levels on Otrin's homeworld were also found to be at 6,000 isorems. (VOY: "Friendship One")

According to the Star Trek Encyclopedia (4th ed., vol. 1, p. 379), the Federation measurement isorem was "based on the old Earth radiation unit of rem, which is an acronym of roentgen equivalent man."