By the way is a phrase people use to add something extra to a conversation, usually something they just remembered or something loosely connected to what they're already talking about. If you're telling your friend about your science project and suddenly remember they left their jacket at your house, you might say, “By the way, you left your jacket here yesterday.”

The phrase signals a small shift in topic without completely changing the subject. It's like opening a side door in the conversation. When your mom asks if you finished your homework and then adds, “By the way, your cousin is visiting this weekend,” she's moving from one topic to another related one.

People also use it to slip in information that might otherwise feel awkward or abrupt. Saying “by the way” softens the transition and makes the conversation feel more natural. Sometimes it introduces something important that the speaker almost forgot: “Oh, by the way, the test got moved to Friday.”

You'll sometimes see it abbreviated as BTW in text messages or online, where people write more casually and look for shortcuts.