Greek alphabet is a letter used in Greek, and it is also widely used in mathematics, physics, biology, astronomy and other disciplines. Greek letters are similar to English letters and Russian letters, but with different symbols and the same phonetic properties. Greek letters are the earliest letters with vowels in the world. Cyrillic letters and Georgian letters used in Russian and Ukrainian are all developed from Greek letters, so people who have studied Russian will feel deja vu when they use Greek letters. Greek letters have entered the vocabulary of many languages. For example, the international word Delta comes from the Greek letter δ, because δ is a triangle.

Theta is a Greek letter, pronounced Sita.

Theta (capital θ, lowercase θ), in Greek, is the eighth Greek letter.