n. 1 the second letter of the Greek alphabet (B, á). 2 a second-class mark given for a piece of work or in an examination. 3 Astron. the second brightest star in a constellation. 4 the second member of a series. beta-blocker Pharm. a drug that prevents the stimulation of increased cardiac action, used to treat angina and reduce high blood pressure. beta particle (or ray) a fast-moving electron emitted by radioactive decay of substances (orig. regarded as rays).
[ ME f. L f. Gk ]