The Hebrew word hallel-u-yah means “All praise God!” Hallel is praise. The u adds “all of us” making it a plural verb, hallelu, let us all praise. Yah, spelled yod-hay, is an abbreviation for YHWH (Yod-Hay-Vav-Hay), the name of God. In Hebrew Scripture it is two words, hallelu yah. Following the Septuagint’s and St. John’s Greek Allelouia, St. Jerome’s Latin Vulgate gives it consistently as Alleluia. That has become the traditional Catholic usage.