The word bomb should be reserved for movies that lose a lot of money (relative to their budgets), not films that break even or lose a bit of money necessarily. Only one in 5 or 10 wide releases make any significant profit in their theatrical runs. Most films break even or lose a little bit of money (that is recuperated eventually by home video and TV rights). The odd film misses the mark entirely and results in 10s of millions (and sometimes hundreds of millions) in losses. Those are bombs.
Interstellar is making big money in China. It opened during the week, but its tally should end up in the $40M range through Sunday. That will put it in position for a $80-100M finish in China.