English word orthopositronium comes from English positronium, English ortho-
| Dictionary entry | Language | Definition |
|---|---|---|
| positronium | English (eng) | (physics) An onium consisting of a positron (anti-electron) and an electron, as a particle–anti-particle bound pair.. (physics, chemistry) An exotic atom consisting of a positron and an electron, but having no nucleus. |
| ortho- | English (eng) | (chemistry) Having an extra proportion of water. (chemistry) in isomeric benzene derivatives, having the two substituents in adjacent positions (compare meta- and para-). (physics) of any molecule of the form X2 in which the two nuclei have parallel spin. Straight, right, proper. |
| orthopositronium | English (eng) | (physics) Used attributively to describe the state of a positronium exotic atom in which the positron and electron have parallel spins. |