Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • adj. Of, relating to, occurring, or performed during half a day.
  • adj. Occurring or coming approximately once every 12 hours, as the tides.
  • adj. Of or relating to the arc described by a celestial body between its meridian passage and its points of rising or setting.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • adj. Twice daily (usually of tides).

from the GNU version of the Collaborative International Dictionary of English

  • adj. Pertaining to, or accomplished in, half a day, or twelve hours; occurring twice every day.
  • adj. Pertaining to, or traversed in, six hours, or in half the time between the rising and setting of a heavenly body.

from The Century Dictionary and Cyclopedia

  • Pertaining to or accomplished in half a day (either twelve hours or six hours); continuing half a day.
  • In entomology, partly diurnal; flying in twilight; crepuscular; specifically, of or pertaining to the Semidiurna.

Etymologies

semi- +‎ diurnal (Wiktionary)

Examples

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