Geometry

1. Half of a circle, formed by cutting a circle along its diameter. Its arc measures 180 degrees.
5. A straight line segment extending from the center of a circle or sphere to its circumference or surface. It is also the length of this segment.
7. A continuous portion of the circumference of a circle (or any other curve).
9. A pattern of one or more geometric shapes that completely covers a plane without any gaps or or overlapping. Also known as tiling.
10. A straight line segment whose endpoints both lie on a circle (or any other curve).
13. A line segment connecting the midpoints of two sides of a triangle. In a trapezoid, it connects the midpoints of the non-parallel sides.
14. A transformation that "flips" a figure over a line (the line of reflection), creating a mirror image.
15. The longest side of a right-angled triangle, opposite the right angle.
18. A straight line segment that passes through the center of a circle or sphere and has its endpoints on the circumference or surface. It is the longest chord of a circle.
19. The point of concurrency of the angle bisectors of a triangle. It is also the center of the triangle's inscribed circle (incircle).

2. In a triangle, a line segment joining a vertex to the midpoint of the opposite side.
3. A transformation that changes the size of a figure without changing its shape. It involves a center point and a scale factor.
4. An angle that measures more than 90 degrees but less than 180 degrees.
6. An angle that measures less than 90 degrees.
8. A transformation that moves every point of a figure or a space by the same distance in a given direction. It's often described as a "slide."
11. The quality of being made up of exactly similar parts facing each other or around an axis; regularity of form or arrangement in terms of balanced proportions. Common types include line (or reflection) symmetry and rotational symmetry.
12. The geometric center of a plane figure. In a triangle, it is the point where the three medians of the triangle intersect.
14. A transformation that turns a figure about a fixed point (the center of rotation) by a specific angle and direction.
16. A line (or segment or ray) that touches a curve or curved surface at exactly one point, without crossing it. For a circle, a tangent line is perpendicular to the radius at the point of tangency.
17. In a triangle, a line segment from a vertex perpendicular to the opposite side (or to the line containing the opposite side). It represents the height of the triangle from that vertex to the base.