These definitions are much more accurate:
https://www.splcenter.org/fighting-hate/extremist-files/ideology/alternative-right
The Alternative Right, commonly known as the Alt-Right, is a set of far-right ideologies, groups and individuals whose core belief is that white identity is under attack by multicultural forces using
political correctness and social justice to undermine white people and their civilization. Characterized by heavy use of social media and online memes, Alt-Righters eschew establishment conservatism, skew young, and embrace white ethno-nationalism as a fundamental value.
http://www.usatoday.com/story/news/politics/onpolitics/2016/11/22/alt-right-trump-white-nationalist-clinton-breitbart-spencer/94273282/
The two groups have similar focuses and goals and white-nationalists can be part of the alt-right movement. But Hawley said the difference is the alt-right movement has no real formal organization and mostly exists online.
I think part of it is more a difference of style and marketing than a difference in substance, though I would note that it seems like most of the leading figures of the alt-right do disavow things like genocide, which some of the more outrageous earlier white nationalists didnt necessarily do," Hawley told the Post.
On September 9, 2016, several leaders of the alt-right community held a press conference, described by one reporter as the "coming-out party" of the little-known movement, to explain their goals. They proclaimed racialist beliefs, stating "Race is real, race matters, and race is the foundation of identity."
Alt-right - Wikipedia
https://en.wikipedia.org/wiki/Alt-right