What punctuation mark is used to indicate a sudden break or abrupt change in thought an unfinished word sentence?

The punctuation mark used to indicate a sudden break or abrupt change in thought, an unfinished word, or sentence is the em dash (—).

Here are some examples:

* "I was about to say something—but then I changed my mind."

* "She started to say—well, never mind."

* "The dog ran into the street—and then..."

The em dash can also be used to set off parenthetical information, although parentheses are more common for this purpose.