In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a proton is transformed into a neutron, or vice versa, inside an atomic nucleus. This process allows the atom to move closer to the optimal ratio of protons and neutrons. As a result of this transformation, the nucleus emits a detectable beta particle, which is an electron or positron.

Beta decay is mediated by the weak force. There are two types of beta decay, known as beta minus and beta plus. In beta minus (β⁻) decay a neutron is lost and a proton appears and the process produces an electron and electron antineutrino, while in beta plus (β⁺) decay a proton is lost and a neutron appears and the process produces a positron and electron neutrino; β⁺ decay is thus also known as positron emission.

An example of electron emission (β⁻ decay) is the decay of carbon-14 into nitrogen-14:

In this form of decay, the original element becomes a new chemical element in a process known as nuclear transmutation. This new element has an unchanged mass number A, but an atomic number Z that is increased by one. As in all nuclear decays, the decaying element (in this case 14
6C) is known as the parent nuclide while the resulting element (in this case 14
7N) is known as the daughter nuclide. The emitted electron or positron is known as a beta particle.

In finance, the beta (β or beta coefficient) of an investment indicates whether the investment is more or less volatile than the market. In general, a beta less than 1 indicates that the investment is less volatile than the market, while a beta more than 1 indicates that the investment is more volatile than the market. Volatility is measured as the fluctuation of the price around the mean: the standard deviation.

Beta is a measure of the risk arising from exposure to general market movements as opposed to idiosyncratic factors. The market portfolio of all investable assets has a beta of exactly 1. A beta below 1 can indicate either an investment with lower volatility than the market, or a volatile investment whose price movements are not highly correlated with the market. An example of the first is a treasury bill: the price does not go up or down a lot, so it has a low beta. An example of the second is gold. The price of gold does go up and down a lot, but not in the same direction or at the same time as the market.