Jonathan, i don't have source where you can read this, i'm talking from experience and using my common sense:
keyword stuffing by google
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in order to be stuffing it must be out of context - if you have a list of categories or related products that contain the keyword, that's not out of context, it's natural.
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also, the same keyword must be repeated over and over, in a compact form, just like in the google example "We sell custom cigar humidors. Our custom cigar humidors are handmade. If you’re thinking of buying a custom cigar humidor, please contact our custom cigar humidor specialists at custom.cigar.humidors@example.com." - this might be the case of some ecommerce and affiliate sites that have thing content, but when most of your occurrence are not coming from the main content (but are coming from the related content, menus, even html), that's not keyword stuffing.
About the keyword density, i have 3 examples, which in terms of density are exactly the same:
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1000 words of content and the keyword appears 20 times per throughout the content
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1000 words of content and keywords appear 20 times in the first 50 words on the page (density is still the same).
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1000 words of content and the keywords appear 10 times in the title and 10 times in the last 30 words (and just for the sake of it, the main content of the article has nothing do with the keywords).
Keyword density is exactly the same for all, but one of them is not keyword stuffing. If it was keyword stuffing in all of the above examples, you wouldn't see ranking any big retailer in the top 10.
So the key to understanding keyword stuffing is if the keywords appear natural, in context, or not. And natural for a blog / content website may be different from natural for an online shop, considering they have a different structure and purpose by default.