Probability is a mathematical measure of how likely events are to occur. It can be expressed as a fraction, decimal, or percentage between 0 and 1. A probability experiment involves possible outcomes that make up a sample space. An event is a subset of outcomes. Theoretical probability calculates the likelihood of an event based on equally likely outcomes. Experimental probability is based on observed frequencies. Subjective probability relies on estimates rather than calculations. As experiments are repeated, experimental probability approaches theoretical probability due to the law of large numbers.