Keywords

Keywords are words and phrases that people type into search engines. Keywords are essential for content marketing and Search Engine Optimization (SEO). They let you discover how people are looking for your products, services or information, and simplify the process of finding them. With the right implementation they help your site rank higher in search results.

Keywords are also used in the campaign tracking parameters:
pk_keyword or utm_keyword

More about reporting of search engines and keywords:
https://help.piwik.pro/analytics/search-engines-keywords/