Probability

Probability is a number between 0 and 1 that measures how likely something is to happen, where 0 means impossible and 1 means certain. A probability of 0.5 means an outcome happens about half the time over many tries, like a fair coin landing heads.

Probability is the language data science uses to reason about uncertainty, and almost every statistical method rests on it. The most useful everyday picture is the long-run frequency: if you repeated the situation a huge number of times, the probability is the fraction of times the event would occur.