Idem is a pointing word meaning “the same one.” English took it from Latin almost unchanged. You will almost never hear it in a café. You will see it in the small notes at the bottom of a page, in law papers, and in old-fashioned book lists, where it saves the writer from repeating a name they have just given. If the last note was by Chen, the next one that is also by Chen — or from the same book — may say idem instead of writing the name again. It means “from the same book, writer, and so on, as the one that has just been mentioned.” Though actually, many writers now just repeat the name, or use ibid. for “in the same work.” Idem still turns up, especially in legal writing. Students do not need this for talk; they need it so a footnote does not look like a mystery. Say it like EYE-dem.