Definitions
from The American Heritage® Dictionary of the English Language, 5th Edition.
 noun The boundary line of a circle.
 noun The boundary line of an area or object.
 noun The length of such a line.
 noun The margin or area surrounding something.
from The Century Dictionary.
 To include in a circular or spherical space.
 noun The line that bounds a circle; by extension, the bounding line of any regular plane curvilinear figure; a periphery: as, the circumference of a circle or an ellipse. The circumference of a sphere is that of a great circle of the sphere.
 noun Hence Loosely, any bounding line: as, the circumference of a city.
 noun The space included in a circle; anything circular in form.
 noun A going about; circumlocution.
from the GNU version of the Collaborative International Dictionary of English.
 transitive verb obsolete To include in a circular space; to bound.
 noun The line that goes round or encompasses a circular figure; a periphery.
 noun A circle; anything circular.
 noun The external surface of a sphere, or of any orbicular body.
from Wiktionary, Creative Commons Attribution/ShareAlike License.
 noun geometry The
line thatbounds acircle or othertwodimensional figure  noun geometry The
length of such a line  noun obsolete The surface of a round or spherical object
 verb obsolete, transitive To
include in acircular space; tobound .
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 noun the boundary line encompassing an area or object
 noun the length of the closed curve of a circle
 noun the size of something as given by the distance around it
Etymologies
from The American Heritage® Dictionary of the English Language, 4th Edition
from Wiktionary, Creative Commons Attribution/ShareAlike License
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Examples

It follows directly from this discussion, that for our spherebeings the circumference of a circle first increases with the radius until the circumference of the universe is reached, and that it thenceforward gradually decreases to zero for still further increasing values of the radius.
Chapter 31. The Possibility of a “Finite” and Yet “Unbounded” Universe 1920

This means the circumference is about 1000 meters, so v is 1000 meters per minute, which yields a simulated gravity about 1/6 of Earth's  the same as the moon (though the people inside move as though the gravity is Earthnormal).
8/1/08: Launch Pad, day 2 prof_brotherton 2008

It makes tubes 44 stitches in circumference or straight pieces up to 44 stitches wide and takes up to a sort of medium chunky weight of yarn  just right for a tubular scarf but a bit narrow to make hats.
Archive 20090101 MadeleineS 2009

It makes tubes 44 stitches in circumference or straight pieces up to 44 stitches wide and takes up to a sort of medium chunky weight of yarn  just right for a tubular scarf but a bit narrow to make hats.
Hurrah for new toys! MadeleineS 2009

Unfortunately that means my jeans are quite uncomforable to wear, because the belt circumference is now at least two holes smaller than the waistband of the jeans.

Lord Howe is a ring of land some one hundred and fifty miles in circumference, several hundred yards wide at its widest, and towering in places to a height of ten feet above sea level.
MAUKI 2010

Paul Rodriguez was a Mexican porter who from the upper and lateral part of his head had a horn 14 cm in circumference and divided into three shafts, which he concealed under a cap.
Human horns frankwu 2010

The atoll of Hikueru lay low on the water, a circle of pounded coral sand a hundred yards wide, twenty miles in circumference, and from three to five feet above highwater mark.
THE HOUSE OF MAPUHI 2010

The park circumference is approximately 40 kilometers (km), extending from the bridge at Ürgüp then leading north for 6 km following the River Damsa.
Göreme National Park and the Rock Sites of Cappodocia, Turkey 2008

Large Hadron collider is 27 Km in circumference and will accelerates protons to 99. 99999% the speed of light (I might not have gotten the right number of 9s, sorry if this spoils your calculations if you are trying this at home).
Boing Boing 2008
fbharjo commented on the word circumference
mercuric fence
January 27, 2016