name a line segment

Oscar?

AB?

http://www.mathsisfun.com/definitions/line-segment.html

Oscar is definitely out of line.

Sure! One example of a line segment is the segment connecting two points, let's call them A and B. In order to name the line segment, you can simply use the names of the two points it connects. So, for example, if the segment connects points A and B, you can name it "AB".