D meson

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of two mesons, one neutral and one positively charged, containing a charm quark and having masses 3,649 and 3,658 times that of an electron and mean lifetimes of about 4.2 × 10⁻¹³ and 1.04 × 10⁻¹² second, respectively.