1. Event - any specific collection of the possible outcomes of a random phenomenon.

2. Frequency of an event - the number of times the event occurs in a sequence of repetitions of the random phenomenon.

3. Relative frequency of an event - the fraction or proportion of repetitions on which the event occurs. A relative frequency is always a number between 0 and 1.

4. Probability of an event - if in a long sequence of repetitions the relative frequency of an event approaches a fixed number, that number is the probability of the event. A probability is always a number between 0 (the event never occurs) and 1 (the event always occurs).