Plain English Meaning
A small line on the glass cockpit display that predicts where your speed, altitude, or heading is heading next, based on how fast it's changing right now.
Full Definition
On an electronic flight display (EFD), trend indicators are short visual markers — typically magenta or cyan lines or arrows — that project where a value such as airspeed, altitude, or heading will be in approximately six seconds if the current rate of change continues. They appear adjacent to the relevant tape or scale on the primary flight display.
Context Anchor
Seen on electronic primary flight displays, often beside the airspeed, altitude, vertical speed, or heading information.
Derivation
From ',' meaning the general direction something is moving. The indicator shows the trend — where the value is going, not just where it is now.