ATS Bible Dictionary
LandmarkFences and walls seem to have been little used in Judea, Mark 2:23, though gardens were sometimes inclosed. The ancient and permanent limits, therefore, of individual property in the open field, Ruth 2:3, were marked by trees or heaps of stones at the corners; and as it was easy, by removing these, to encroach on a neighbor's ground, a peculiar form of dishonesty arose, requiring a severe punishment, De 19:14 Proverbs 22:28 Hosea 5:10. 
International Standard Bible Encyclopedia
LANDMARKland'-mark (gebhul, literally, "boundary"): The boundary may have been marked, as at present, simply by a furrow or stone. The iniquity of removing a landmark is frequently insisted on (Deuteronomy 19:14; Deuteronomy 27:17 Proverbs 22:28; Proverbs 23:10 Job 24:2 gebhulah), its removal being equivalent to theft.
Easton's Bible Dictionary
A boundary line indicated by a stone, stake, etc. (
Deuteronomy 19:14; 
27:17; 
Proverbs 22:28; 
23:10; 
Job 24:2). Landmarks could not be removed without incurring the severe displeasure of God. 
Webster's Revised Unabridged Dictionary
1. (
n.) Any conspicuous object on land that serves as a guide; some prominent object, as a hill or steeple.
2. (n.) A mark to designate the boundary of land; any , mark or fixed object (as a marked tree, a stone, a ditch, or a heap of stones) by which the limits of a farm, a town, or other portion of territory may be known and preserved.
Strong's Hebrew
1366. gebul -- border, boundary, territory... border, bound, coast, great, 
landmark, limit, quarter, space. 
... The territory inclosed --
 border, bound, coast, X great, 
landmark, limit, quarter, space. 
... /hebrew/1366.htm - 6k 1367. gebulah -- border, boundary
... border, bound, coast, landmark. Or (shortened) gbulah {gheb-oo-law'}; feminine of
 gbuwl; a boundary, region -- border, bound, coast, landmark. Place. ... 
/hebrew/1367.htm - 6k