In a horizontal direction or position.

(genetics) The transfer of genetic material from one organism to another one that is not its offspring; especially common among bacteria.

(geology) A form of graded bedding that occurs at the bottom of a body of water. The principle says that smaller colloids (particles) settle farther in the strata while larger particles settle closer to the source of water.

The property of being horizontal.

(gymnastics) A horizontally-aligned bar used in gymnastics, upon which acts of swinging are performed. / (gymnastics) A gymnastics event using the horizontal bar.

horizontalness

A cable run installed inside a building wall.

(idiomatic, euphemistic) Sexual intercourse.

plural of horizontalist

An advocate or believer in horizontalism.