Hashem is a Hebrew term for God. Literally, it means “the name.” In the Bible the Hebrew word for God is made up of four vowels, and according to tradition it was only pronounced on Yom Kippur by the High Priest. Saying God’s name was considered a very serious and powerful thing, so much so that one of the Ten Commandments prohibits us from saying God’s name in vain. As a result, people have come up with various substitutions.
When reading Torah, we generally substitute the word Adonai for the four letter un-pronounceable name of God. Outside of reading Torah and praying, God is often referred to as Hashem, a creative way of not saying God’s name.
There are many other names for God in Jewish tradition, including
Adoshem, Yah, Yahweh, HaKadosh Baruch Hu, El Shaddai, Av Harahamim, and Harahaman.