But what is a delimiter?

Answer 1

All symbols used to mark both the start and end of a region of text, like () "" '' <> {} etc.. But also those that mark a border, like commas, and end of line, like semi-colons.

Answer 2

The above is sort of correct.

A delimiter is any character in a string used to separate values.

EX: "This,is,a,sentence,that,uses,commas,as,delimiters"
"This.is.a.sentence.that.uses.periods.as.delimiters."
"This is a sentence that uses spaces as delimiters"

Answer 3

Jay Welborn's answer makes the best sense.