Definition of Key word stuffing?

Once the same keyword is used repeatedly on a webpage it turns into keyword stuffing. If you do so it will bring your webpage top on search results. But you won’t get any benefits from Google and other search engines.

So, place proper keywords at essential place with respect to the contents of your webpage without repeating keywords which is better than keyword stuffing.
Stuffing your keywords can create many bad effects, which links some spam to your web pages and contents. When keyword exceeds its limit, search engines spam your webpage permanently.

So if you want to avoid keyword stuffing, replace all keywords with best synonyms where you can serve better. Don’t repeat the keywords again and again in a webpage of a website.