Plain English Meaning
A short line on a glass-cockpit display that shows where your speed, altitude, or heading is heading next if you keep doing what you're doing right now.
Full Definition
A graphical line or arrow displayed on an electronic flight instrument (such as a primary flight display) that predicts where a parameter — typically airspeed, altitude, or heading — will be a set number of seconds in the future if the current rate of change continues.
Context Anchor
Seen on electronic flight displays and moving-map displays, especially when monitoring changes in airspeed, altitude, vertical movement, or aircraft path.