Plain English Meaning
A flat tool with degree markings around its edge that lets you measure the angle between two lines. On a pilot's plotter, it's the curved scale you use to read the direction of your planned route off the chart.
Full Definition
A measuring instrument, usually semicircular or circular and marked in degrees from 0 to 180 or 0 to 360, used to measure or plot angles. On an aeronautical plotter, the protractor portion is used to measure the true course of a route by placing its center on a meridian line on a sectional chart and reading the angle the course line makes with that meridian.
Context Anchor
Seen when using an aviation plotter on a chart during flight planning.
Derivation
From Latin protractus, meaning 'drawn ' or 'extended.' The instrument 'draws out' an angle into something measurable in degrees.