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Its like how a Square is a Square but also Rhombus and a Rectangle. Long tail keywords can be any and all of those definitions depending on the moment, your goals and your needs. In most cases I wouldn't consider a Low Traffic term a "Long Tail Keyword" simply because it has low traffic. But I would argue that Long Tail Keywords are likely to have lower traffic than their short tail counterparts. People are more likely to search using shorter terms and phrases but sometimes a 4+ word combination can be popular. But I would argue that a Long Tail Keyword/term/phrase is likely to be 3, 4, 5+ words long. Can those words be all extremely product specific? Sure. Do they need to be? No. Are they more specific than a one or two word short tail phrase? Yes. One word with low traffic is probably just a bad keyword to be using. A 4 word phrase with high traffic could just be a popular long tail search. In some cases it will depend on your industry. Some phrases can have multiple meanings. Just because a 4 word phrase has good traffic doesn't mean you need to use it if the first 3 pages in the SERPs are all sites devoted to a completely different concept that happens to have the same phrasing.