Definition of "theta"

The eighth letter of the Modern Greek alphabet, ninth in Old Greek: Θ, θ.  examples

(mathematics) The measure of an angle.  examples

(aviation, by extension) Pitch angle; the angle between an aircraft's longitudinal axis and the horizontal plane.  examples

(finance) The sensitivity of the value of a derivative with respect to time; the "time decay".  examples

(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).  quotations

(phonology) The voiceless dental fricative represented by θ.