Some version of the idea of "relax, take a deep breath" is common across cultures. But what does the "deep" in "deep breath" mean to people? I've come across three plausible interpretations, and I'm curious what people colloquially mean (I suspect it's a mix or conflation of the meanings).

Deep as in slow (low number of breaths per minute)

One interpretation of "deep" is "slow", from the idea that "going deep" into something takes time. So, deep breathing just means slow breathing. This is particularly relevant to somebody who's doing a lot of breaths per minute: "relax, take a deep breath" means "slow down."

Deep as in large (high breath volume)

Another interpretation I've seen of "deep" is "large" -- take a lot of good air in, and expel all the bad air out. Get a fresh start by cleansing the bad air from inside you.

The Johns Hopkins page on hyperventilation uses the term "deep breathing" (in a critical fashion) equating it to rapid, large breaths.

Deep as in diaphragmatic

Here, the term "deep" seems to be referring to the location of the trigger for breathing (the diaphragm is deeper down/inside in the body compared to the upper chest?).

This is the most scientifically "correct" definition of deep breathing; see for instance Wikipedia or Harvard Health Publishing.

All three?

See this WebMD article that seems to combine all three meanings: slow, large, and diaphragmatic. A lot of breathing instruction and direction I see in popular culture does seem to combine all three elements when using the term "deep breathing".

Are the meanings in conflict?

Generally speaking, diaphragmatic breathing is likely to be slower, and slower breathing is likely to be diaphragmatic, so those meaning are not in conflict, though they still mean different things.

However, the relationship between breath rate and breath volume is very unclear, with any of the 2 X 2 combinations possible.

In particular, it's very much possible to take breaths that are "shallow" in terms of breath volume, while being slow and diaphragmatic, and in fact the whole idea of "reduced breathing" (as seen in things like the Buteyko method) is to try to reduce total breathing volume per unit time by doing some combination of slow breathing (reducing the rate of breathing) and "shallow" breathing (in the sense of reduced breathing volume). In this viewpoint, then, the "slow" part is good but the "large" part is bad in a way that cancels out the slow part. So the ideal is "slow, small"; both "fast, small" and "slow, large" are worse (as they mean more breathing volume per minute), and "fast, large" is the worst (hyperventilation at its worst.

The 2 X 2 from a Buteyko-style reduced breathing perspective (which I believe to approximately match my scientific understanding so far, though I'm still learning):

	slow	fast
small	best	mid
large	mid	worst

On the other hand, if your theory of breathing is the colloquial one where you're trying to "cleanse" yourself as much as possible with each breath, your 2 X 2 would look like this:

	slow	fast
small	mid	worst
large	best	mid

So I'm curious how people interpret the term "deep breathing" and how they couple or decouple different potential interpretations!

Thanks to Issa Rice for some helpful discussions that led me to this post. Issa did not review the post.