A conjunction meaning 'while saying ...' (doing something at the same time as speaking) or used in the pattern '...と言いながら...' to indicate a contrast between what is said and what is actually done/feel.
彼は謝りながらも、どこか楽しそうだった。
He apologized, but at the same time he somehow looked pleased.
『行かない』と言いながら、結局ついて来た。
Saying 'I'm not going,' he ended up coming along after all.
新聞を読みながらコーヒーを飲んだ。
I drank coffee while reading the newspaper.