Orthopositronium was a form of positronium, an atom-like system consisting of an electron and a positron. As the electron and positron were oppositely charged, the particle behaved like a hydrogen atom; however, the positron had a much smaller mass than the proton it replaced. As the electron and positron were anti-particles of each other, any form of positronium was very unstable and its lifespan was measured in nanoseconds.
Orthopositronium was used to coat the mirrors of Mariah Learman's time machine. (AUDIO: The Time of the Daleks)