The term "semicircumference" is used, especially in older works (e.g., Cayley 1891, 1897) to refer to half of a circumference.
In terms of the circumference, radius, and diameter of a circle, the semicircumference
is given by
Cayley, A. "On Some Problems of Orthomorphosis." J. reine angew. Math.107, 262-277, 1891.Cayley, A. "On
Some Problems of Orthomorphosis." In The Collected Mathematical Papers of
Arthur Cayley, Vol. 13. Cambridge, England: Cambridge University Press,
pp. 198 and 200, 1897.
, radius 
, and diameter 
of a circle, the semicircumference
is given by
