noun

• (mathematics) The measure of an angle.
• (aviation, by extension) Pitch angle; the angle between an aircraft's longitudinal axis and the horizontal plane.
• (finance) The sensitivity of the value of a derivative with respect to time; the "time decay".
• (biology) 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."

noun

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