Anything that violates Google's Webmaster guidelines could fall in negative SEO Link farms (linking sites within same IP) over stuffing keywords, copied content etc too are looked down by search engines!
Some people use negative SEO to refer to tactics used to spoil your competitor's ranking so that they drop down the SERPs rather than you having to overtake them.
Negative SEO can boost a site to the top of Google search results for lucrative search terms, and at the same time reduce a competing site’s visibility to searchers or making it seem to disappear from search results altogether.
Negative SEO is also know as google bowling. It means sabotaging your competitors to get them out of the way
In general negative SEO is about doing harm to the site or page. But I think that ineffective SEO can be considered as negative too. I mean when you spend more money on SEO in comparison with the resulting income in a long-run period.