Abbreviation for "Trade Cards By Mail". Often used by Pokemon TCG Youtubers who trade cards to other players or Pokemon Youtubers in other cities/states/countries. Both parties send their cards, protected, in an envelope to each other and then post youtube videos about their successful TCBMs.
This is a video going over my current TCBMs with user007, user008, and user009.
I got some cool cards I needed for my deck in this TCBM with user007.