An intrinsic nature, essence or characteristic that is unique to some phenomena that can be described as that phenomena's self.
I said this in one of my answers but was scored down by the same Mahayana who believe in these ideas.
An intrinsic nature, essence or characteristic that is unique to some phenomena CANNOT be described as that phenomena's self.
That a rock is 'hard' by nature is not its "self". That Nibbana is "peaceful" is not its "self". Such ideas are crazy. "Self" is "ego" & "possessiveness" ("I-making" & "mine-making").
“Rāhula, the interior earth element is said to be 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 proper understanding like this: ‘This is not mine, I am not this, this is not my self.’ When you really see with proper understanding, you reject the earth element, detaching the mind from the earth element.
MN 62