The "Championship Belt" concept comes from boxing/wresting and is the idea that every season, the previous years champion (here the Super Bowl) starts with the belt, and each time they lose they pass the belt to the winning team who becomes the new sole owner. 

https://grantland.com/the-triangle/introducing-the-nba-regular-season-championship-belt/

So far this season, the Chiefs have lost this "belt" to the Lions in Week 1 (no, the preseason did not count), who then lost the "belt" to the Seahawks in Week 2. The Seahawks successfully defended their belt against the Panthers Week 3, and thus 

the Seahawks start this market "holding the belt"

 (but will pass it on after their first loss, if they lose).

This market closes at the conclusion of the regular season, resolving simply with the team who holds the belt after Week 18. I'll update each week with the current owner of the belt. As a fun edge case, a tie would resolve in a division into two different belts (and could resolve for multiple teams).

Note that additional answers will not be added, so all teams not listed are contained in "Other". 

Updated Market (that you can add answers to) is here: