ortho

Noun

ortho (countable and uncountable, plural orthos)

1. (countable) An isomer of a benzene derivative having two substituents adjacent on the ring.
   • When the cinnamic acid had been nitrated the ortho was separated from the para, by suspending in alcohol and leading hydrochloric acid gas into the mixture : […].1887 December 21, Thomas M. Morgan, “On Some Derivatives of Ortho-Nitro-Cinnamic Acid”, in Chemical News^([269])
   • Similarly, it also does not make sense to call the spin states of ortho and para hydrogen entangled.2009 February 27, P. Neumann et al., “Response to Comment on "Multipartite Entanglement Among Single Spins in Diamond"”, in Science, volume 323, number 5918, →DOI
2. (countable) A certain type of flat eyepiece.
   • Orthos are excellent eyepieces, still preferred by some backyard astronomers for medium- and high-power applications, especially planetary observing.1998, Terence Dickinson, NightWatch: A Practical Guide to Viewing the Universe, page 76
3. (countable, dated) An orthochromatic plate.
   • Wherever the exposure is likely to be cut short— and this includes all high speed work and most portraiture — the ortho is not so suitable as the ordinary plate.1908 June, Charles F. Rice, “On the Use of Orthochromatic Plates”, in Photographic Times, page 174