theta

noun6

1. •
   The eighth letter of the Modern Greek alphabet, ninth in Old Greek: Θ, θ.
2. •
   The measure of an angle.
3. •
   Pitch angle; the angle between an aircraft's longitudinal axis and the horizontal plane.
4. •
   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. — 2018
5. •
   The voiceless dental fricative represented by θ.
6. •
   The sensitivity of the value of a derivative with respect to time; the "time decay".

noun1

1. •
   (US, informal) A member of the Kappa Alpha Theta sorority.