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If they are abbreviations which are extremely common to the field, once per work is often enough to define them. If they are rare, invented for the piece, or really jargon, I would say once per sec...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/1694 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
If they are abbreviations which are extremely common to the field, once per work is often enough to define them. If they are rare, invented for the piece, or really jargon, I would say once per section (once per chapter, once per web page). Alternatively, a list of acronyms at the beginning or end of a piece might also be helpful.