Micromorts are a measure riskiness of activities. One micromort is a one-in-a-million chance of death.
Of course, someone has catalogued a bunch of activities with the micromorts associated with each. Scuba diving, for example, is 5 micromorts (or 0.0005% chance of death) per dive. Skydiving is 10 micromorts (0.001%) per dive. Attempting to climb Mount Everest is 37,932 micromorts (3.79%), which is honestly lower than I would have expected.