Incidentally means by the way, or as a side point not directly related to the main topic. When you're talking about one thing and suddenly remember something loosely connected, you might say “incidentally” before mentioning it.

If you're explaining to your friend how you studied for the math test, you might add, “Incidentally, did you know Mr. Chen used to teach in Japan?” The word signals you're briefly stepping away from your main point to share an extra detail.

Teachers use it when adding information that's interesting but not essential: “The Mississippi River is 2,340 miles long. Incidentally, it's also one of the oldest rivers in the world.” Writers use it to tuck in bonus facts without disrupting their main argument.

The word suggests the information came up naturally or by chance while discussing something else. It's different from “importantly” or “significantly,” which demand attention. Incidentally is more casual, almost like saying “oh, and another thing” or “speaking of that.”

Sometimes people use it when what follows isn't actually incidental at all, but they want to sound casual about delivering important news: “Incidentally, I won first place in the science fair.”