theta

plural thetas

1. The eighth letter of the Modern Greek alphabet, ninth in Old Greek: Θ, θ.
2. mathematicsThe measure of an angle.
3. broadlyaeronauticsPitch angle; the angle between an aircraft's longitudinal axis and the horizontal plane.

   Synonyms: deck angle

4. businessThe sensitivity of the value of a derivative with respect to time; the "time decay".
5. biologyDesignating 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 September 19, Meghan O'Gieblyn, “The Most Unread Book Ever Acclaimed”, in The Paris Review

6. human sciencesThe voiceless dental fricative represented by θ.

plural Thetas

1. UScolloquialA member of the Kappa Alpha Theta sorority.