1) By the by 2) Bythebye 3) By the way 4) Oh by the way 1) By the bye 2) By the way 3) In an incidental manner 4) In passing 5) Introducing a different topic 6) Parenthetically 
[adv] - in an incidental manner
apart or aside from the main subject of attention, discussion, etc.; by the way; parenthetically. · in an incidental manner.