theta

noun

1. The eighth letter of the Modern Greek alphabet, ninth in Old Greek: Θ, θ.
2. The measure of an angle.
3. (broadly)Pitch angle; the angle between an aircraft's longitudinal axis and the horizontal plane.
4. The sensitivity of the value of a derivative with respect to time; the "time decay".
5. Designating a level of brain activity as measured by electroencephalography having a frequency of between four and seven cycles per second (associated with drowsiness in adults).

   “The reader is less likely to throw the book down in a fit of disgust than she is to be lulled into a theta state, a highway hypnosis induced by page after page of incantatory prose.”

6. The voiceless dental fricative represented by θ.
7. (US, colloquial)A member of the Kappa Alpha Theta sorority.