1.  boundary(n = noun.location) bound, bounds - the line or plane indicating the limit or extent of something;
is a kind of extremity
has particulars: hairline, frontier, heliopause, border, borderline, boundary line, delimitation, mete, bourn, bourne, district line, county line, city line, border, edge, end, demarcation, demarcation line, limit, lineation, outline, surface, shoreline, rubicon, moho, mohorovicic discontinuity
Derived form verb bound₂

2.  boundary(n = noun.shape) bound, edge - a line determining the limits of an area;
is a kind of line
has particulars: rim, border, margin, perimeter, fringe, outer boundary, periphery, brink, threshold, verge, upper bound, lower bound, thalweg
Derived form verb bound₂

3.  boundary(n = noun.attribute) bound, limit - the greatest possible degree of something; "what he did was beyond the bounds of acceptable behavior"; "to the limit of his ability"
is a kind of extent
has particulars: knife-edge, absoluteness, starkness, utterness, heat barrier, thermal barrier, level best, maximum, utmost, uttermost, brink, verge

   That which indicates or fixes a limit or extent, or marks a bound, as of a territory; a bounding or separating line; a real or imaginary limit. [1913 Webster]
"But still his native country lies
Beyond the boundaries of the skies." [1913 Webster]
" That bright and tranquil stream, the boundary of Louth and Meath." [1913 Webster]
"Sensation and reflection are the boundaries of our thoughts." [1913 Webster]

boundary, n. (pl. -ies)
1 a line marking the limits of an area, territory, etc. (the fence is the boundary; boundary between liberty and licence).
2 Cricket a hit crossing the limits of the field, scoring 4 or 6 runs.

Idiom:
boundary layer the fluid immediately surrounding an object that is immersed and moving. boundary rider Austral. & NZ a person employed to ride round the fences etc. of a cattle or sheep station and keep them in good order. boundary umpire (in Australian Rules) an umpire on the sidelines who signals when the ball is out.

Etymology:
dial. bounder f. BOUND(2) + -ER(1) perh. after limitary

N  limit, boundary, bounds, confine, enclave, term, bourn, verge, curbstone, but, pale, reservation, termination, terminus, stint, frontier, precinct, marches, backwoods, boundary line, landmark, line of demarcation, line of circumvallation, pillars of Hercules, Rubicon, turning point, ne plus ultra, sluice, floodgate, definite, conterminate, conterminable, terminal, frontier, bordering, thus far, thus far and no further, stick to the reservation, go beyond the pale, Special.

boundary condition, boundary layer, boundary line, cretaceous-tertiary boundary, outer boundary, state boundary