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.
semi- + diurnal (Wiktionary)