I read the following on the internet:
I think that in Buddhism the word refers to elements of experience -- and that the words represent subjective or perceived qualities of a thing. For example, I don't know, ice might be hard and wet and not-hot so it seems to be composed or (or is perceived as having) those two elements.
Does "dhatu" in Pali refer to "elements of experience"?
As it was explained to me, dhatu means something like "aspect" or "tier". Meaning, it's a cross-cutting principle found in a whole that's delineated when you analytically disect the whole in a particular way.
So, when you analyze experience, you may delineate "the visual aspect" - present across many rather different experiences. When you analyze external dharmas, you may delineate "the aspect of solidity" present across many very different objects etc.
Today we could delineate e.g. the electromagnetic aspect, the chemical aspect, the informational aspect. All are elements or principles of the whole observed as different "analytical planes".
ChrisW is not wrong to explain it as he did for the purpose of contrasting dhatu with the chemical elements of the periodic table. They are subjective inasmuch as it takes an act of analysis to delineate and "focus" on one dhatu at a time. Of course they are also objective because they are actual aspects of an ontological whole.
As part of the gradual instruction, Buddha taught basic phenomenological analysis, delineating dhatus such as "the seen", "the heard" etc.
The culmination of his teaching, though, is the notion of amrita-dhatu aka dharmadhatu, the most fundamental aspect of everything. Seeing this dhatu is seeing Nirvana. This dhatu can be described as the absolute reference point, substratum of all fluctuation.
I think that MN 140 for example says that "earth" refers to anything hard and solid.
And what is the earth element? The earth element may be interior or exterior. And what is the interior earth element? Anything hard, solid, and organic that’s internal, pertaining to an individual. This includes head hair, body hair, nails, teeth, skin, flesh, sinews, bones, bone marrow, kidneys, heart, liver, diaphragm, spleen, lungs, intestines, mesentery, undigested food, feces, or anything else hard, solid, and organic that’s internal, pertaining to an individual. This is called the interior earth element. The interior earth element and the exterior earth element are just the earth element. This should be truly seen with right understanding like this: ‘This is not mine, I am not this, this is not my self.’ When you truly see with right understanding, you reject the earth element, detaching the mind from the earth element.
Those things are components of a body, of which their solidity seems to me an experiential quality.
In context I think they may describe not only experience but "aggregates of clinging" -- the text quoted above includes upādinna.
I wrote "elements of experience" in the bit you quoted in the question because in that answer I hoped to clarify that's not the same as a chemical 'element' (like Hydrogen and so on).
There's a dictionary definition e.g. here suggests its meaning is similar to "element" in English: meaning both a "physical element" (i.e. an element of something physical) and/or an element of something else.
It might also be translated as "a sort of" or "a type of" (except that's not idiomatic English).
