The Tetragrammaton (Greek meaning word with four letters) is the usual reference to the Hebrew name for God, which is transliterated from the Hebrew as YHWH -- four consonants with no vowels; it is the distinctive personal name of the God of Israel. The popular vocalized form of YHWH is Yahweh.
In Judaism, the Tetragrammaton is the ineffable name of God, and is not read aloud. In the reading aloud of the scripture or in prayer, it is replaced with Adonai (Lord). Other written forms such as ד׳ or ה׳ are read as Hashem (the Name), for the same reason.
One theory regarding the disuse of the Tetragrammaton is that the Jewish taboo on its pronunciation was so strong that the original pronunciation may have been lost somewhere in the first millennium. Since then, many scholars (particularly Christians) have sought to reconstruct its original pronunciation. For example, circa 1518 Christian theologians introduced the pronunciation Yehovah based on the written form with the vowel pointing for Adonai. This is generally held to be implausible but gave rise to alternate vocalizations such as Jahovah and Jehovah.
The theory regarding the disuse of the Tetragrammaton (YHWH) is the result of an interpretation of the Third of the Ten Commandments. The Jewish people stopped saying the Name by the 3rd century out of fear of violating the commandment "You shall not take the name of YHWH your God in vain" (Exodus 20:7).
It is possible that the practice was in effect prior to early Christian times as Jesus prayed to the Father "I have made your name known" (John 17:26).
According to one Jewish tradition, the Tetragrammaton is related to the causative form, the imperfect state, of the Hebrew verb הוה (ha·wah, "to be, to become"), meaning "He will cause to become" (usually understood as "He causes to become").
Another tradition regards the name as coming from three different verb forms sharing the same root YWH, the words HYH haya (היה): "He was"; HWH howê (הוה): "He is"; and YHYH yihiyê (יהיה): "He will be". This would therefore show that God is timeless and self-existent. Other interpretations include the name as meaning "I am the One Who Is." This can be seen in the O. T. biblical account of the "burning bush" commanding Moses to tell the sons of Israel that "I AM (אהיה) has sent you," (Exodus 3:13-14). Some suggest, "I AM the One I AM" [אהיה אשר אהיה]. This may also fit the interpretation as "He Causes to Become."