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-on 1
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suff.
1.
a. A discrete, small unit, such as a subatomic particle: baryon.
b. A property of a physical system when quantized as a particle: phonon.
2. A sequence of nucleotides or region of DNA: codon.

[From (ELECTR)ON (influenced by Greek -on, neuter of -os, n. and adj. suff., and -on, neuter of -ōn, n. and adj. suff.)]

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2016 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
 
-on 2
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suff.
Inert gas: radon.

[New Latin, from (ARG)ON.]

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2016 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
 
-on 3
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suff.
A chemical compound that is not a ketone or a compound that contains oxygen in a carbonyl group: parathion.

[Alteration of -ONE.]

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2016 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
 
ON
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abbr.
1. Old Norse
2. Ontario

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2016 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
 
on (ŏn, ôn)
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prep.
1.
a. Used to indicate position above and supported by or in contact with: The vase is on the table. We rested on our hands and knees.
b. Used to indicate contact with or extent over (a surface) regardless of position: a picture on the wall; a rash on my back.
c. Used to indicate location at or along: the pasture on the south side of the river; a house on the highway.
d. Used to indicate proximity: a town on the border.
e. Used to indicate attachment to or suspension from: beads on a string.
f. Used to indicate figurative or abstract position: on the young side, but experienced; on her third beer; stopped on chapter two.
2.
a. Used to indicate actual motion toward, against, or onto: jumped on the table; the march on Washington.
b. Used to indicate figurative or abstract motion toward, against, or onto: going on six o'clock; came on the answer by accident.
3.
a. Used to indicate occurrence at a given time: on July third; every hour on the hour.
b. Used to indicate the particular occasion or circumstance: On entering the room, she saw him.
4.
a. Used to indicate the object affected by actual, perceptible action: The spotlight fell on the actress. He knocked on the door.
b. Used to indicate the object affected by a figurative action: Have pity on them.
c. Used to indicate the object of an action directed, tending, or moving against it: an attack on the fortress.
d. Used to indicate the object of perception or thought: gazed on the vista; meditated on his actions.
5. Used to indicate the agent or agency of a specified action: cut his foot on the broken glass; talked on the telephone.
6.
a. Used to indicate a medicine or other corrective taken or undertaken routinely: went on a strict diet.
b. Used to indicate a substance that is the cause of an addiction, a habit, or an altered state of consciousness: high on dope.
7.
a. Used to indicate a source or basis: "We will reach our judgments not on intentions or on promises but on deeds and on results" (Margaret Thatcher).
b. Used to indicate a source of power or energy: The car runs on methane.
8.
a. Used to indicate the state or process of: on leave; on fire; on the way.
b. Used to indicate the purpose of: travel on business.
c. Used to indicate a means of conveyance: ride on a train.
d. Used to indicate availability by means of: beer on tap; a physician on call.
9. Used to indicate belonging to: a nurse on the hospital staff.
10. Used to indicate addition or repetition: heaped error on error.
11.
a. Concerning; about: a book on astronomy.
b. Concerning and to the disadvantage of: We have some evidence on him.
12. Informal In one's possession; with: I haven't a cent on me.
13. At the expense of; compliments of: drinks on the house.
adv.
1. In or into a position or condition of being supported by or in contact with something: Put the coffee on.
2. In or into a position of being attached to or covering something: Put your clothes on.
3. In the direction of something: He looked on while the ship docked.
4.
a. Toward or at a point lying ahead in space or time; forward: The play moved on to the next city.
b. At or to a more distant point in time or space: I'll do it later on.
c. Toward or to a different state or condition: Let's move on to another subject.
5. In a continuous course: He worked on quietly.
6.
a. In or into performance or operation: Turn on the radio.
b. In progress or action; in a state of activity: The show must go on.
7. In or at the present position or condition: stay on; hang on.
8. In a condition of being scheduled for or decided upon: There is a party on tonight.
adj.
1. Being in operation: The television is on.
2.
a. Engaged in a given function or activity, such as a vocal or dramatic role: You're on in five minutes!
b. Under or behaving as if under observation: A minister is always on.
3. Informal Functioning or performing at a high degree of competence or energy: The goalie is really on.
4.
a. Planned; intended: We have nothing much on for this weekend.
b. Happening; taking place: The parade is on.
5. Baseball Having reached base safely; on base: Two runners are on.
Idioms:
be on to Informal
To be aware of or have information about: You'll never deceive us again; we're on to you.
on and off
Intermittently.
on and on
Without stopping; continuously.

[Middle English, from Old English an, on; see an- in the Appendix of Indo-European roots.]

Usage Note: Both on and onto can be used to indicate motion toward a position: The cat jumped on the table or The cat jumped onto the table. However, onto is more specific, indicating that the motion was initiated from an outside point. They wandered onto the battlefield means that they began their wandering at some point off the battlefield, while They wandered on the battlefield implies that the wandering began on the battlefield.

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2016 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
 

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