Black Hat SEO is the process of Improve the website ranking and traffic in inorganic way.Google consider the SPAM of work so dont use it.

- Duplicate content.
- Hidden text
- Keyword stuffing
- Doorway pages
- Cloaking
- Paid links

Using Spamy techniques and not following google webmaster guidelines are consider as Black Hat

Ex: Keyword Stuffing in content

- Duplicate content
- Hidden text
- Paid links
- Doorway pages
- Cloaking
- Keyword Stuffing

Black hat SEO refers to the use of forceful SEO techniques that focus only on search engines and not a human audience, and usually does not obey search engines rules.

Spamdexing is the deliberate manipulation of search engine indexes. It involves a number of methods, such as link building and repeating unrelated phrases, to manipulate the relevance or prominence of resources indexed,

What is Black Hat SEO?
 

Black hat SEO is a practice against search engine guidelines, used to get a site ranking higher in search results. These unethical tactics don't solve for the searcher and often end in a penalty from search engines. Black hat techniques include keyword stuffing, cloaking, and using private link networks.