Plain English Meaning
A line drawn on a performance chart that tells you when to stop following one set of curves and start following the next. It's the handoff point between one step of the calculation and the next.
Full Definition
On a performance chart, a reference line is a vertical line that marks the starting point for working through a variable section of the chart. The pilot enters the chart on the left, follows the curves until reaching the reference line, and then begins moving along the next set of curves to account for the next variable (such as weight, wind, or obstacle height).
Context Anchor
Seen on takeoff performance charts when a pilot is working across several chart sections to find a takeoff distance.
Derivation
From ',' meaning a fixed point you measure or from. On a chart with several variables, the reference line is the agreed point where the chart-maker says, 'start the next step here.'