ちなみに is a discourse marker that introduces an additional, often supplementary, piece of information that is related to the current topic but not the main point. It is similar to saying 'by the way' or 'incidentally' in English. It is often used when you remember something that might be useful or interesting to the listener, even if it is not directly asked for. It can also be used to add a detail that supports or clarifies what was just said.

In conversation, ちなみに is common and neutral in politeness, but it is slightly more common in semi-formal or informational contexts. It can be used in both speaking and writing, but it is not as abrupt as 'by the way' in casual English; it often sounds a bit more planned. You can place it at the beginning of a sentence or after a comma, and it does not change the grammar of the rest of the sentence.