Borrowed from Middle French horizontal.

• (RP) IPA: /ˌhɒɹɪˈzɒntəl/
• (America) IPA: /ˌhoɹəˈzɑntəl/

horizontal

1. Perpendicular to the vertical; parallel to the plane of the horizon; level, flat.

   horizontal lines

2. (marketing) Relating to horizontal markets
3. (archaic) Pertaining to the horizon.
   • 1667, John Milton, “Book I”, in Paradise Lost. […], London: […] [Samuel Simmons], and are to be sold by Peter Parker […]; [a]nd by Robert Boulter […]; [a]nd Matthias Walker, […], →OCLC ↗; republished as Paradise Lost in Ten Books: […], London: Basil Montagu Pickering […], 1873, →OCLC ↗, lines 594-595:

     As when the Sun new ris'n / Looks through the Horizontal misty Air

4. (wine) Involving wines of the same vintages but from different wineries.
5. (music, of an interval) Having the two notes sound successively.

   Synonyms: linear, melodic

   Antonyms: vertical

6. (sex, euphemism) Relating to sexual intercourse.

   horizontal tango

7. (biology) Being or relating to the transmission of organisms between biotic and/or abiotic members of an ecosystem that are not in a parent-progeny relationship.

   Infectious agents may spread by horizontal transmission.

• vertical

• French: horizontal
• German: horizontal, waagrecht, waagerecht
• Italian: orizzontale
• Portuguese: horizontal
• Russian: горизонта́льный
• Spanish: horizontal

horizontal (plural horizontals)

1. A horizontal component of a structure.
2. (geology) Horizon.
3. A Tasmanian shrub or small tree whose main trunk tends to lean over and grow horizontally, Anodopetalum biglandulosum