|"Is this all real? Or has this been happening inside my head?"
The subject of this article is a real-life person, company, product, or creative work that has been mentioned "in-universe" in a canon source. The Harry Potter Wiki is written from the perspective that all information presented in canon is true (e.g., Hogwarts really exists), and, as such, details contained in this article may differ from real world facts.
King William III of England (14 November 1650 – 8 March 1702) was King of England from 1689, ruling jointly with his wife Queen Mary II until her death in 1694. He reigned alone until his own death in 1702.
When the British Ministry of Magic was begging for recognition and protection under Muggle law, the couple failed to send a delegation to them, which contributed to the introduction of the International Statute of Wizarding Secrecy in 1692. 
The name "William" is from the Germanic name Willahelm, which was composed of the elements wil "will, desire" and helm "helmet, protection".
- J.K. Rowling's Official Site (Mentioned only)