Horizontal signifies in the direction of or parallel to the horizon. For practical purposes level and horizontal are identical, tho level, as the more popular word, is more loosely used of that which has no especially noticeable elevations or inequalities; as, a level road.

J. C. Fernald, English Synonyms and Antonyms (1914)

Example sentences

linear Tom loves non-linear differential equations.
melodic The wind chimes sound melodic now and then.
plane In culture, they are on the same plane as savages.

Popularity over time

Word	1850 → 2019	Trend
horizontal		falling
linear		steady
melodic		steady
plane		steady

Use in printed books, per decade from 1850 to 2019.

By length

4 letters: even², flat¹
5 letters: level², plane¹, plain¹
6 letters: linear³
7 letters: melodic³
9 letters: recumbent

The small number after each word is its number of syllables (even² = 2 syllables).

In other languages

Français: horizontal
Italiano: orizzontale
Deutsch: horizontal