Concise Bible Dictionary:

This word, signifying “witness,” is added in the AV in Joshua 22:3434And the children of Reuben and the children of Gad called the altar Ed: for it shall be a witness between us that the Lord is God. (Joshua 22:34). Instead of “called the altar Ed,” it has been translated “gave a name to the altar.” The word “Ed” is in some Hebrew MSS, and in the Syriac and Arabic versions, but not in the LXX.

Jackson’s Dictionary of Scripture Proper Names:

a witness