Noun

(en noun)

(history of science) A hypothetical particle posited by Greek philosophers as an ultimate and indivisible component of matter.

(physics, chemistry) The smallest possible amount of matter which still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons.

* {{quote-magazine, year=2013, month=September-October, author= Katie L. Burke

, magazine=(American Scientist), title= In the News , passage=Oxygen levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis:

A mote of dust in a sunbeam.

The smallest, indivisible constituent part or unit of something. (Now generally interpreted as a figurative use of the physics sense, above.)

* 1835 , Sir , Sir (James Clark Ross), Narrative of a Second Voyage in Search of a North-west Passage …'', Volume 1 , pp.284-5

Towards the following morning, the thermometer fell to 5°; and at daylight, there was not an atom of water to be seen in any direction.

A very small amount (of something immaterial); a whit.

* 1873 , (Isabella Macdonald Alden) (as "Pansy"), Three People , Western Tract and Book Society (1873), page 325:

"I have hardly the faintest atom of hope," answered this honest, earnest man.

*

those of us who are capable of it are forced to work to the last atom of our strength

(mathematics) A non-zero member of a (Boolean algebra) that is not a union of any other elements.

(computing, programming, Lisp) An individual number or symbol, as opposed to a list. A scalar value.

Noun

(en noun)

A group of four things.

(chemistry) A tetravalent atom or radical.

(biology) Two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis process.

(biology) Group of four haploid and immature pollen grains in tetrahedral fashion produced by meiotic microsporogenesis.

("tetrad" on Wikipedia)