We first see this name in the Bible in Genesis 21:33.
After peacefully resolving an issue over water-rights with a local tribal leader, Abraham plants a tree to commemorate the event. There he calls God El-Olam. Olam comes from a root word meaning eternity and the word literally means forever or unchanging. El is a Hebrew word and name for God we've discussed before. Combining the two words creates a title for God that shows us a beautiful facet of His character: we translate El-Olam as "The Everlasting God" or "The Eternal God."
I believe Abraham uses this name because he has found that God, eternal and unchanging, is completely reliable in perilous situations. This name is also found in Jeremiah 10:10 and Isaiah 26:4.