Likelihood
Hazmat Term
is expressed as either a frequency or a probability. Frequency is a measure of the rate at which events occur over time (e.g., events/year, incidents/year, deaths/year, etc.). Probability is a measure of the rate of a possible event expressed as a fraction of the total number of events (e.g., one-in-a-million, 1/1,000,000, or 1X10-3).
Source:

Hazardous Materials Safety term