"that which indicates the limits of anything," 1620s, from bound (n.1) + -ary. Strictly, a visible mark indicating a dividing line, a bound being the limit or furthest point of extension of any one thing.
c. 1300, "boundary marker," from Anglo-Latin bunda, from Old French bonde "limit, boundary, boundary stone" (12c., Modern French borne), variant of bodne, from Medieval Latin bodina, which is perhaps from Gaulish.
From mid-14c. as "an external limit, that which limits or circumscribes;" figuratively, of feelings, etc., from late 14c. From late 14c. as "limits of an estate or territory." Now chiefly in out of bounds, which originally referred to limits imposed on students at schools; the other senses generally have gone with boundary.
adjective and noun word-forming element, in most cases from Latin -arius, -aria, -arium "connected with, pertaining to; the man engaged in," from PIE relational adjective suffix *-yo- "of or belonging to." The neuter of the adjectives in Latin also were often used as nouns (solarium "sundial," vivarium, honorarium, etc.). It appears in words borrowed from Latin in Middle English. In later borrowings from Latin to French, it became -aire and passed into Middle English as -arie, subsequently -ary.
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Definitions of boundary from WordNet
the line or plane indicating the limit or extent of something;