incidentally是表示“换题”的衔接词,意为“随便说及地”,说话者用以引入临时想到补充要说的事情。

Incidentally,I think you still owe me some money.顺便说一句,我想你还欠我一些钱。
Incidentally, why don't you sent me a photograph of yourself?顺便提一下,你为什么不寄给我一张你的照片呢?

［同义词］   by the way