Different therapists and researchers (and farmers, actually) use the word rumination in different ways, and that makes it important to define exactly what I mean when I use that word.
When I talk about rumination, I’m talking about any type of mental engagement with the problem; put another way, I’m talking about shifting into problem-solving mode. This includes analyzing, mental reviewing, mental checking, visualizing, monitoring, and even directing attention toward the problem.* Crucially, all of these mental processes are controllable. They don’t happen to us; we do them on purpose.**
I group all of the above mental processes together not only because they serve a similar function and are controllable, but because they are all characterized by a subjective experience of mental engagement with the problem. I believe it is easier to see the totality of rumination when you think about it as mental engagement with the problem, broadly speaking, rather than several discrete mental processes.
Without a broad enough definition of rumination, aspects of this compulsion persist unnoticed, and continue to cause anxiety. But my purpose is not to argue semantics. Call it rumination or call it quidditch, mental engagement with the problem is what I am talking about when I talk about rumination.
The above definition of rumination excludes: a thought (or vague image) occurring to you; knowing; and noticing. These mental phenomena are not aimed at solving the problem, they are not controllable, and they are not the problem. It’s the mental engagement with these thoughts, the effort to ‘solve’ them, that is the problem.
Thus, according to this definition:
A thought occurring to you? Not rumination.
But trying to figure out what the thought means or whether to believe it or how to make it go away? Rumination.
Knowing you have a distressing problem or question? Not rumination.
But trying to solve or answer it? Rumination.
Noticing something? Not rumination.
But monitoring it or directing attention toward it? Rumination.
A vague image occurring to you? Not rumination.
But visualizing something? Rumination.
It is absolutely crucial for therapists and patients to know exactly where the line is between uncontrollable mental processes and controllable ones because OCD is largely a product of trying to control the mental processes you can’t control, while failing to control the ones you can and should control. And helping a patient to make this distinction and to adjust their strategy accordingly is, in my opinion, a foundation of effective treatment. To put this in terms of ERP, it is crucial to accurately distinguish between the obsession and the compulsion.
As I’ve said elsewhere, I believe that rumination, as defined above, is the cornerstone of OCD, as well as all other anxiety disorders. I therefore believe that teaching a person to recognize and eliminate rumination is essential to effective treatment. And a prerequisite for this intervention is a definition of rumination that is broad enough to capture all manifestations of this phenomenon.
*More or less anything that would be included in Kahneman’s System 2: https://en.wikipedia.org/wiki/Thinking,_Fast_and_Slow
**Compulsive ruminators feel as though they can’t stop, but in reality they’re just conflicted. There is indeed a part of them that wants to stop, but there’s another part of them that is trying to solve the problem, and these are mutually exclusive goals.