theta
noun
the·ta ˈthā-tə 
chiefly British ˈthē-tə
1
: the 8th letter of the Greek alphabet see Alphabet Table
2
: theta rhythm
Word History
Etymology
Middle English, from Latin, from Greek thēta, of Semitic origin; akin to Hebrew ṭēth teth
First Known Use
15th century, in the meaning defined at sense 1