Collinear set of points is a set of points all of which lie on the same straight line.

Non-collinear set of points is a set of THREE or more points that do not all lie on the same straight line.   (Remember, you can connect any two points with a line.)

Distance between two points on the real number line is the absolute value of the difference of the coordinates of the two points.   Distance is always a positive number (or zero.)

B is between A and C if and only if A, B, and C are distinct collinear points (different points) and AB + BC = AC

Line segment (or Segment)  is a set of points consisting of two points on a line, called endpoints, and all of the points on the line between the endpoints.  i.e. it's a piece of a line with two ends.  

Length or Measure of a line segment is the distance between its endpoints.

Congruent segments are segments that have the same measure. i.e. They have the same length.

Midpoint of a line segment is a point of that line segment that divides the segment into two congruent segments. 

Bisector of line segment is any line, or subset of a line (ray, line segment) that intersects the line segment at its midpoint.

Half-line is a set of points on one side of a point.

Ray is a part of a line that consists of a point on the line, called an end-point, and all the points on one side of the endpoint.

Opposite Rays are two rays of the same line with a common endpoint and no other poitn in common.  i.e. They start in the same place, but go in opposite directions.

Angle is a set of points that is the union of two rays having the same endpoint

Straight Angle  is an angle that is the union of opposite rays and whose degree measure is 180.

Acute angle is an angle whose degree measure is greater than 0 and less than 90.

Right Angle  is an angle whose degree measure is 90.

Obtuse Angle is an angle whose degree measure is greater than 90 and less than 180.

Congruent Angles are angles that have the same measure.

Bisector of an Angle is a ray whose endpoint is the vertex of the angle and that divides that angle into two congruent angles.

Perpendicular Lines are two lines that intersect to form right angles.

Distance from a point to a line is the length of the perpendicular from the point to the line.

Polygon is a closed figure in a plane that is the union of line segments such that the segments intersect only at their endpoints and no segments sharing a common endpoint are collinear.

Triangle is a polygon that has exactly three sides.

Scalene Triangle is a triangle that has no congruent sides.

Isosceles Triangle is a triangle that has at least two congruent sides.

Equilateral Triangle is a triangle that has three congruent sides.

Acute Triangle is a triangle that has three acute angles.

Right Triangle is a triangle that has a right angle.

Obtuse Triangle is a triangle that has an obtuse angle.

Equiangular Triangle is a triangle that has three congruent angles.