Scrum Guide
The official, freely available document defining Scrum, maintained by Ken Schwaber and Jeff Sutherland, most recently updated in 2020.
What it means
The Scrum Guide is intentionally short, around 13 pages, defining only the minimal rules of Scrum: its theory, values, events, artifacts, and accountabilities. Everything beyond that is left to teams to fill in with their own practices.
The 2020 update removed prescriptive language in places, renamed 'Development Team' to 'Developers,' folded the Scrum Master's daily-management language into servant leadership, and formalized the Product Goal and Sprint Goal as commitments.
Common mistakes
Because it is the authoritative source, any Scrum certification, book, or blog post that contradicts the current Scrum Guide should be treated as opinion or practice, not official Scrum.
Example
A certification exam question about 'Development Team' would be based on an older Scrum Guide version, since the current one uses 'Developers.'