EPHRAIM, MOUNT ē’ frĭ əm (הַר־אֶפְרָֽיִם, occurs thirty-two times in the OT, Josh 17:15; Judg 3:27; 1 Sam 1:1; etc.). Since it denotes the hill country in central Pal. occupied by the tribe of Ephraim, rather than a single mountain, it is better rendered “the hill country of Ephraim” (as ASV, RSV). It was more fruitful than Judaea, esp. on its western slopes and it was one of the few areas where the Israelites were able to establish themselves after the conquest under Joshua. For this reason the two main sanctuaries of the Judges’ period, Bethel and Shiloh, were within its borders.

International Standard Bible Encyclopedia (1915)

(har ’ephrayim):

Means that part of the mountain which fell to Ephraim (Jos 19:50, etc.). The natives speak today of Jebel Nablus, Jebel Cafed, etc., meaning that section of the central range which is subject to each city. It is better therefore to retain the rendering of the King James Version, and not to read with the Revised Version (British and American) "hill-country of Ephraim."