1 the third letter of the Greek alphabet.
2 a third-class mark given for a piece of work or in an examination.
3 Astron. the third brightest star in a constellation.
4 the third member of a series.
Phrases and idioms:
gamma radiation (or rays) electromagnetic radiation of very short wavelength emitted by some radioactive substances.
Etymology: ME f. Gk
Useful english dictionary. 2012.