Theta

noun

1. The eighth letter of the Modern Greek alphabet, ninth in Old Greek: Θ, θ.
2. The measure of an angle.
3. The sensitivity of the value of a derivative with respect to time; the "time decay".
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).
5. The voiceless dental fricative represented by θ.