level (n.)

mid-14c., "tool to indicate a horizontal line," from Old French livel "a level" (13c.), ultimately from Latin libella "a balance, level" (also a monetary unit), diminutive of libra "balance, scale, unit of weight" (see Libra). Spanish nivel, Modern French niveau are from the same source but altered by dissimilation.

Meaning "position as marked by a horizontal line" (as in sea-level) is from 1530s; meaning "flat surface" is from 1630s; meaning "level tract of land" is from 1620s. Figurative meaning in reference to social, moral, or intellectual condition is from c. 1600. Figurative phrase on the level "fair, honest" is from 1872; earlier it meant "moderate, without great ambition" (1790).

level (adj.)

early 15c., "having an even surface," from level (n.). Meanings "lying on or constituting a horizontal surface" and "lying in the same horizontal plane" (as something else) are from 1550s. To do one's level best is U.S. slang from 1851, from level in the sense "well-aimed, direct, straight." Level playing field as a figure of equality of opportunity is from 1981. Related: Levelly.

level (v.)

mid-15c., "to make level" (transitive), from level (n.). From c. 1600 as "to bring to a level." Intransitive sense "cease increasing" is from 1958. Meaning "to aim (a gun)" is late 15c. Slang sense of "tell the truth, be honest" is from 1920. To level up "to rise" is attested by 1863.

A word here as to the misconception labored under by our English neighbor; he evidently does not understand the American manner of doing things. We never level down in this country; we are always at work on the up grade. "Level up! Level up!" is the motto of the American people. [James E. Garretson, "Professional Education," in "The Dental Cosmos," Philadelphia, 1865]

Modern use is mostly from computer gaming (2001). To level off "cease rising or falling" is from 1920, originally in aviation. Related: Leveled; leveling.

Definitions of level
level (n.)
a position on a scale of intensity or amount or quality;
a high level of care is required
Synonyms: degree / grade
level (n.)
a relative position or degree of value in a graded group;
Synonyms: grade / tier
level (n.)
a specific identifiable position in a continuum or series or especially in a process;
Synonyms: degree / stage / point
level (n.)
height above ground;
the pictures were at the same level
the water reached ankle level
level (n.)
indicator that establishes the horizontal when a bubble is centered in a tube of liquid;
Synonyms: spirit level
level (n.)
a flat surface at right angles to a plumb line;
park the car on the level
Synonyms: horizontal surface
level (n.)
an abstract place usually conceived as having depth;
a good actor communicates on several levels
Synonyms: layer / stratum
level (n.)
a structure consisting of a room or set of rooms at a single position along a vertical scale;
what level is the office on?
Synonyms: floor / storey / story
level (v.)
aim at;
level criticism or charges at somebody
level (v.)
tear down so as to make flat with the ground;
The building was levelled
Synonyms: raze / rase / dismantle / tear down / take down / pull down
level (v.)
make level or straight;
level the ground
Synonyms: flush / even out / even
level (v.)
direct into a position for use;
Synonyms: charge / point
level (v.)
talk frankly with; lay it on the line;
I have to level with you
level (v.)
become level or even;
The ground levelled off
Synonyms: level off
level (adj.)
of the score in a contest;
Synonyms: tied / even
level (adj.)
having a surface without slope, tilt in which no part is higher or lower than another;
acres of level farmland
Synonyms: flat / plane
level (adj.)
not showing abrupt variations; "she gave him a level look"- Louis Auchincloss;
spoke in a level voice
Synonyms: unwavering
level (adj.)
being on a precise horizontal plane;
a billiard table must be level
level (adj.)
oriented at right angles to the plumb;
the picture is level