In Wicked Problems, Righteous Solutions, published in 1991, Peter DeGrace and Leslie Stahl call scrum an "all-at-once model." It refers to the rugby scrum, where players work together to move the ball upfield, a few steps at a time, in a huddle. The authors mention it in passing but don't give it detail.
Sutherland and Schwaber fill in the detail, using scrum as a tool to solve a problem at Easelcorp – where, as with Individual, requirements and priorities change so fast that developers can't get anything done. Sutherland introduces the idea of a sprint: An uninterrupted period of time during which the team can develop and test without change. This sprint has a fixed length, starting at 90 days. After the sprint, the team will "demo" the software to customers, who react to what was built and come up with ideas for the next sprint. This slows the feedback process for the development team from the customers, typically referred to as the larger, "outer circle" of scrum. After the sprint, the team also meets to discuss what worked, what didn't work and what to change. This is called a retrospective.
The inner circle of scrum, meanwhile, is the daily scrum, an opportunity for the team to discuss work in progress in order to accelerate team performance. The meeting is called a scrum because it resembles the rugby 'huddle.' The term originates in the Harvard Business Review.
Since those initial definitions, the ceremonies of scrum remain the daily scrum (or "standup"), the demo and the retrospective (or "review"), along with a planning meeting to discuss what to build.