Also see definition of "Jah" in Word Study
the everlasting
JAH [smith]
(Jehovah), the abbreviated form of Jehovah, used only in poetry. It occurs frequently in the Hebrew, but with a single exception, (Psalms 68:4) is rendered "Lord" in the Authorized Version. The identity of Jah and Jehovah is strongly marked in two passages of Isaiah-- (Isaiah 12:2; 26:4) [
Also see definition of "Jah" in Word Study