The American Heritage® Dictionary of the English Language, 4th Edition
n. A symbol ( — ) used in writing and printing to indicate a break in thought or sentence structure, to introduce a phrase added for emphasis, definition, or explanation, or to separate two clauses.
"Making sure \expndtw-2 that everything is delivered on time and put in place is \expndtw-1 something John does\emdash which is why he's busy now and \expndtw-2 we're not."