From the verb עדר (adar), to miss or fail, or rather from the noun עדר (eder), a flock or herd.
The name Eder is part of a group of similar names that were pronounced slightly differently (other names in this group are Ader and Edar).

The names in the Bible are:

• A son of Mushi, son of Merari, son of Levi, son of Jacob, the arch-father of Israel (1Chronicles 23, 23)
• A city located in southern Judah (Joshua 15, 21)