There are a few simple rules. The shorter the better. Make it catchy or easy to remember. Make it relevant to your niche or industry. The fewer words within the domain the better. Finding a domain which is 1 word is really hard these days (for a .com domain), 2 words, is more feasible. 3 words are borderline, and anything more should have a really special reason but I suggest against it. Unless it is extremely memorable. Additionally...
A short introduction to the basic elements that make your website attractive to search engines.
Try broadening your search.
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