As a ~~pediatric~~ pedantic but fun point, in surveying, level lines are defined to follow the Earth's curvature. This means they are technically not straight. Horizonal lines, which are perpendicular to plumb lines, are often used as approximations for level lines.
Generally it's safe to use them interchangeably, but sometimes the distinction is important.
https://theconstructor.org/surveying/terms-in-leveling-uses/20077?amp=1
