The best replacement for line segment is line.

The best alternatives to line segment are rated as following:
★★★★★  line
★★★★  ray
★  segment
  an angle

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Speaking about line segment, one probably shouldn't forget about midpoint.

Other search terms related to line segment and their ratings are:
★★★★★ midpoint
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Google Trends

Scale is based on the average worldwide traffic of line segment in all years. Click on graph for more.
line segment
line
ray