Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. A half of a circle as divided by a diameter.
 n. An object or arrangement of objects or people in the shape of half a circle.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. Half of a circle.
 n. An instrument for measuring angles.
from the GNU version of the Collaborative International Dictionary of English
 n.
 n. The half of a circle; the part of a circle bounded by its diameter and half of its circumference.
 n. A semicircumference.
 n. A body in the form of half of a circle, or half of a circumference.
 n. An instrument for measuring angles.
from The Century Dictionary and Cyclopedia
 n. The half of a circle; the part of a circle comprehended between a diameter and the half of a circumference; also, the half of the circumference itself.
 n. Any body or arrangement of objects in the form of a halfcircle.
 n. An instrument for measuring angles; a species of theodolite with only half a graduated circle; a graphometer.
 n. In meteorology, one half of the area covered by a cyclonic storm. In the northern hemisphere the dangerous semicircle for navigation is that on the righthand side as one advances along the path of the center of a progressive storm: the manageable, or navigable, or safer semicircle is that on the lefthand side.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. a plane figure with the shape of half a circle
Etymologies
Examples

According to the theorem, any triangle inscribed in a semicircle is a right triangle, as is shown in the following diagram:

The mind would then only tend to the affirmation of the sole motion of a semicircle, which is not contained in the conception of a semicircle, and does not arise from the conception of any cause capable of producing such motion.

The first figure is a semicircle, which is perpendicular to the plane of the circle ABDZ and which starts on the diameter AED and, with point A remaining fixed, rotates to position AKD.

Therefore it is the assumption of B, the half of two right angles, from which it follows that A is attributable to C, i.e. that the angle in a semicircle is a right angle.

The half of a circle is termed a semicircle; thus, in Figure 52, A B and

The vaulting, windows, and doorways had the form of the round arch, that is, a semicircle, as in the ancient Roman monuments.

I have counted wrong; and of those not three are of other material than stone, nor are there as many others that fail to cause expense to your Majesty and labor to the natives  and this in one legua about the city as I have said, in a semicircle, which is even not entire, for the other half falls within this bay.

In the centre of the semicircle was a great round hole, 30 ft. in diameter  an extinct crater.

We had delicious young chicken  quite babies they were, poor dears  fried with cream; and wreathed all round our plates in a semicircle were a quantity of tiny dishes.

Within the semicircle were a number of Arabs of high and low degree, a few of them welldressed and armed to the teeth, others dirty and shabby; but all intent on business.
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