What is Boundary?

Legal Definition
A unit of real estate or immovable property is limited by a legal boundary (sometimes also referred to as a property line or a lot line). The boundary (in Latin: limes) may appear as a discontinuation in the terrain: a ditch, a bank, a hedge, a wall, or similar, but essentially, a legal boundary is a conceptual entity, a social construct, adjunct to the likewise abstract entity of property rights.

A cadastral map displays how boundaries subdivide land into units of ownership. However, the relations between society, owner, and land in any culture or jurisdiction is conceived of in terms more complex than a tessellation. Therefore, the society concerned has to specify the rules and means by which the boundary concept is materialized and located on the ground.

A 'Western' version of the operationalization might be a legally specified procedure, performed by a chartered surveyor, supported by statements from neighbors and pertinent documents, and resulting in official recording in the cadastre as well as boundary markings in the field. Alternatively, indigenous people represent boundaries through ephemeral performances, such as song and dance, and, when in more permanent form, e.g. paintings or carvings, in artistic or metaphorical manner.
-- Wikipedia
Legal Definition
Estates. By this term is understood in general, every separation, natural or artificial, which marks the confines or line of division of two contiguous estates. 3 Toull. n. 171.

2. Boundary also signifies stones or other materials inserted in the earth on the confines of two estates.

3. Boundaries are either natural or artificial. A river or other stream is a natural boundary, and in that case the centre of the stream is the line. 20 John. R. 91; 12 John. R. 252; 1 Rand. R. 417; 1 Halst. R. 1; 2 N. H. Rep. 369; 6 Cowen, R. 579; 4 Pick. 268; 3 Randolph's R. 33 4 Mason's R. 349-397.

4. An artificial boundary is one made by man.

5. The description of land, in a deed, by specific boundaries, is conclusive as to the quantity; and if the quantity be expressed as a part of the description, it will be inoperative, and it is immaterial whether the quantity contained within the specific boundaries, be greater or less than that expressed; 5 Mass. 357; 1 Caines' R. 493; 2 John. R. 27; 15 John. 471; 17 John. R. 146; Id. 29; 6 Cranch, 237; 4 Hen. & Munf. 125; 2 Bay, R. 515; and the same rule is applicable, although neither the courses and distances, nor the estimated contents, correspond with such specific boundaries; 6 Mass. 131; 11 Mass. 193; 2 Mass. 380; 5 Mass. 497; but these rules do not apply in cases where adherence to them would be plainly absurd. 17 Mass. 207. Vide 17 S. & R. 104; 2 Mer. R. 507; 1 Swanst. 9; 4 Ves. 180; 1 Stark. Ev. 169; 1 Phil. Ev. Index, h. t.; Chit. Pr. Index, h. t.; 1 Supp. to Ves. jr. 276; 2 Hill. Ab. c. 24, §209, and Index, h. t.

6. When a boundary, fixed and by mutual consent, has been permitted to stand for twenty-one years, it cannot afterwards be disturbed. In accordance with this rule, it has been decided, that where town lots have been occupied up to a line fence between them, for more than twenty-one years, each party gained an incontrovertible right to the line thus established, and this whether either party knew of the adverse claim or not; and whether either party has more or less ground than was originally in the lot he owns. 9 Watts, R. 565. See Hov. Fr. c. 8, p. 239 to 243; 3 Sum. R 170 Poth. Contr. de Societe, prem. app. n. 231.

7. Boundaries are frequently marked by partition fences, ditches, hedges, trees, &c. When such a fence is built by one of the owners of the land, on his own premises, it belongs to him exclusively; when built by both at joint expense, each is the owner of that part on his own land. 5 Taunt. 20. When the boundary is a hedge and a single ditch, it is presumed to belong to him on whose side the hedge is, because he who dug the ditch is presumed to have thrown the earth upon his own land, which was alone lawful to do, and that the hedge was planted, as is usual, on the top of the bank thus raised. 3 Taunt. 138. But if there is a ditch on each side of the hedge, or no ditch at all, the hedge is presumed to be the common property of both proprietors. Arch. N. P. 328; 2 Greenl. Ev. §617. A tree growing in the boundary line is the joint property of both owners of the land. 12 N. H. Rep. 454.

8. Disputes arising from a confusion of boundaries may be generally settled by an action at law. But courts of equity will entertain a bill for the settlement of boundaries, when the rights of one of the parties may be established upon equitable grounds. 4 Bouv. Inst. n. 3923.
-- Bouviers Law Dictionary
Legal Definition
By boundary is understood, in general, every separation, natural or artificlal, which marks the confines or line of division of two contiguous estates. Trees or hedges may be planted, ditches may be dng, walls or inclosures may be erected, to serve as boundaries. But we most usually understand by boundaries stones or pieces of wood inserted in the earth on the confines of the two estates. Civ. Code La. art. 826. Boundaries are either natural or artificial. Of the former kind are water-courses, growing trees, beds of rock, and the like. Artificial boundaries are landmarks or signs erected by the hand of man, as a pole, stake, pile of stones, etc.
See also
-- Black's Law Dictionary
Legal Definition
The making or bounding line dividing two parcels of land. See 25 L. R. A. (N. S.) 649.
-- Ballentine's Law Dictionary