pixiv (lowercase 'p') is a Japanese artwork site, and one of many major sources of Danbooru posts.
Use this tag only for images that use the pixiv logo or refer to pixiv itself. Don't use it just for images from pixiv.
You can search pixiv:id
(e.g. pixiv:92186303) to find Danbooru posts from a pixiv ID. Search pixiv:any to find all pixiv posts on Danbooru.
For details on using pixiv, see howto:pixiv.