Becquerel and others observed three clearly different types of radiation, but no one knew
exactly what any of them were, so they were simply named alpha (a),
beta (b), and gamma (g), for the first
three letters of the Greek alphabet. It was only later that the radiation was shown to consist
of familiar things: helium nuclei, electrons, and high-energy electromagnetic waves.
goes back to the early days of research in radioactivity, which was
first discovered in 1896 by Henri Becquerel.