Keyword proximity is the distance between a search term's individual keywords. For example, the target keyword "affiliate marketing" in a webpage's title "How affiliates can excel in marketing". The keyword proximity between "affiliate" and "marketing" is 3.
The closer the individual keywords are, of course, the more weight they have to search engines.
It's good to take note of it, but it's not all important in the grand scheme of things. If you're targeting a specific set of keywords, then that's what should appear on your title and within your article. Keyword proximity helps when it's grammatically impossible to do this.
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