An indivisible unit of work.  See also AcidCommand.

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I believe one can have an "Atomic Unit" of many different things.  An "Atomic Unit of Work" is the 
smallest unit of work (in some given context).  The "Atomic Unit of Beer" would be the smallest 
quantity of beer (perhaps, somewhere around a pint).
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Feel free to refactor or remove this "dumb question": isn't there also something in the name atomic which links the idea of state change to that of the ACID transaction (i.e. the transaction is all-or-nothing, you commit or rollback, there is no in-between state, as with electrons - or is my memory of school physics completely off? ''Exactly right! (unless my memory is also gone...)'').
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People often equate the idea of an atom with being 'impossible to divide'.  However, it's useful to remember instead that an atom is divisible, but the pieces are not the same stuff that the whole was. 
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There is a difference between "atom" and "atomic".  The first is a noun, the latter, an adjective.  

For example, in the physical sciences, an "atom" is a particular collection of sub-atomic particles that are tightly bound together; there are similar uses of the word atom in other fields to refer to things that are nominally indivisible, or at least difficult to sub-divide.  

"Atomic", as a modifier, can be applied to a wide variety of kinds of things (ex. "atomic unit of work", "atomic database transaction", etc.) to indicate the indivisibility of the thing referred to.

Just FYI, the etymology of "atomic" is: Middle English, from Latin atomus, from Greek atomos, from atomos indivisible
	see: http://m-w.com/cgi-bin/dictionary?book=Dictionary&va=atomic
	http://m-w.com/cgi-bin/dictionary?book=Dictionary&va=atom