Definition of horizontal
1a
: of or relating to the apparent junction of earth and sky : situated near the horizon
2
: relating to, directed toward, or consisting of individuals or entities of similar status or on the same level
horizontal mergers horizontal hostility
3
medical
: relating to or being transmission (as of a disease) by physical contact or proximity in contrast with inheritance
Genes are normally inherited from parents, but they can also be inserted into a genome by viruses, plasmids, and other foreign agents—a phenomenon called horizontal transfer.— Stephan Reebs
— compare vertical sense 4
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Merriam-Webster unabridged
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Time Traveler for horizontal
The first known use of horizontal was in 1555
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