As CP says -- it's literally where you place a speech balloon so that it creates a tangent in the proper sense of the word:

Note that it's also frowned upon in the art itself. In Klaus Janson's excellent
DC Guide to Inking, he points out a panel where the circle of a gun muzzle touches the panel border and cites it as a rare example of when it's OK for an inker to unilaterally 'fix' the artwork.
Cheers!
Jim