Probability
When we ask ourselves the question "how certain am I that a specific event will occur?", then we are interested in probability. This is very much the same as asking the question 'what is the risk that this will happen?".
Probability can be expressed as a percentage, permillage, or in other words: a proportion.
A related, yet slightly different concept is odds.
Root > Assessing the burden of disease and risk assessment > Field Epidemiology > Measurement in Field Epidemiology > Measures of Disease Occurrence > Odds