what is keyword density in seo ?

[Jake]

Peter has it right, just to add keyword density was always a guessing game as to how densely you could pack in a keyword before it wold be considered unnatural manipulation by the search engines, since Google's renewed interest in site quality, SEO methods from 5 years ago may not work any longer or may actually get a page penalized.

The hummingbird things seems mainly to do with accepting more natural language in search queries, but be assured keyword targeting is not dead, it's still at the heart of their paid sponsored search Adwords system.

http://searchengineland.com/google-hummingbird-172816

It's better to write first for your readers and not worry so much about manipulating the search engines, it would pay to have a target list of keywords to target when creating content, but just use them where it naturally fits.