What does “bring to a simmer” mean?

Question

First, a confession: I work in software, so I'm probably paying way too much attention to the state of liquid that is "a simmer". That written, I love to cook, and no recipe direction gives me more confusion, sadness, and googling than "bring to a simmer". Accept no substitute. I find this to be the most vague direction in all of culinary science, and it drives what's left of my organized mind insane.

So here's the setting. I'm making vichyssoise, because I'm intrigued by the possibility of making a dish that has no color at all. I've been instructed to "bring to a boil and simmer the soup for 35 minutes."

The internet is filled with unsatisfying and at times contradictory answers. My research yields a few prototypical examples:

1. "Simmer" means "low or off position," suggesting basically no heat at all.
2. To "simmer" is to heat to a temperature point just off boiling, generally acknowledged as somewhere around 95 degrees C or something like 195 degrees F.
3. "Simmer" is something like a "soft boil," a vague state that appears to be between "not bubbling" and "roiling", but which by definition must boil in some way, since you know, it's bubbling.

Each of these examples mean fundamentally different things. As far as I can tell, a "simmer" is a phase transition whereby the suspension in question, whatever the soup, sauce, or solid (apparently you "simmer" bratwurst, you never boil it) may be, cooks in a way that only years of experience or training can identify. Hence, my question:

What does "simmer" mean? Does it differ per recipe or is it universally defined?

EDIT: Did a bad copy/paste job from another window.

Answer

Personally, I would argue that 2 and 3 are actually the same, and they are your answer.

If you heat a pan of water you'll notice the bubbles forming before the water is actually boiling, hence the talk of between not bubbling and full on roiling.

Also, when you're making your soup, it isn't pure water, so the boiling temp will not be a perfect 100 degrees C in any case.

So, I would say, that simmering is when you keep it just under a full boil. Watch what you're cooking, there should be gentle movement, but not a full roiling pan of whatever it is you're cooking.

To get something simmering away, you need to bring up to a full boil, then reduce the heat until you're getting movement, but not full bubbling.

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