Probability
1. Probability is the likelihood that an event will occur.
  
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2. Extent to which an event is likely to occur.
Notes (1) : ISO 3534-1:1993, definition 1.1 gives the mathematical definition of probability as “a real number in the scale of 0 to 1 attached to a random event. It can be related to a long-run relative frequency of occurrence or to a degree of belief that an event will occur. For a high degree of belief, the probability is near 1.”
Notes (2) : Frequency rather than probability may be used to describe risk.
Notes (3) : Degrees of belief about probability can be chosen as classes or ranks, such as
- Rare/unlikely/moderate/likely/almost certain, or
- Incredible/improbable/remote/occasional/probable/frequent.
(Source: ISO 22399:2007 – Societal Security - Guideline for Incident Preparedness and Operational Continuity Management) - clause 3.28
3. The chance of a risk occurring.
(Source: Business Continuity Institute - BCI)
4. A measure of the chance of occurrence expressed as a number between 0 to 1.
(Source: Australia. A Practitioner's Guide to Business Continuity Management HB292 - 2006 )