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Alpha (uppercase/lowercase Α α), is the first letter of the Greek alphabet, used to stand for the "a" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 1. Letters that came from it are the Roman A and Cyrillic А.
Alpha is often used in physics and chemistry as a symbol or in the names of things, for example alpha particles (which are two protons and two neutrons bound together into a particle the same as a helium nucleus). The symbol is also used a lot in mathematics in algebra, for example naming angles.