theta

noun

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