Plain English Meaning
A clear plastic ruler with a built-in protractor and chart-matched distance markings. You lay it on a paper chart to draw a course line, read the direction of that line in degrees, and measure how far it is from one point to another.
Full Definition
A handheld navigation tool used in flight planning, combining a straightedge with a protractor scale and one or more distance scales matched to common aeronautical chart scales (typically sectional and world aeronautical charts). It is used to measure true course between two points on a chart and to measure distance along that course.
Context Anchor
Seen during preflight planning when drawing or checking a route on a paper or electronic chart.