Probability is a measure of the likelihood that an event will occur, expressed as a number between 0 and 1.
Probability of an event = number of favorable outcomes / total number of possible outcomes. A probability of 0 means impossible, 1 means certain. Key concepts include independent events, conditional probability, Bayes' theorem, and probability distributions (binomial, normal, Poisson). Probability is essential in statistics, risk assessment, gambling, weather forecasting, quantum mechanics, and machine learning.
Probability originated in 17th-century gambling problems. Blaise Pascal and Pierre de Fermat exchanged letters (1654) about the problem of points, laying the foundation. Christiaan Huygens wrote the first book on probability in 1657. Jacob Bernoulli discovered the law of large numbers. Pierre-Simon Laplace systematized probability theory in the 18th-19th centuries.