Refers to a literal straight line, often used in geometry, drawing, or describing a direct path.
定規を使って一直線を引いてください。
Please draw a straight line using a ruler.
この道は駅まで一直線だ。
This road goes straight to the station.
He headed straight for the goal.
The road stretches in a straight line all the way to the horizon.
The dog ran straight to its owner.