Definition
The mark is the defender directly on the player with the disc. Their job is to apply a Force, count the stall, and physically narrow the throwing options the thrower has available. They are the pressure that turns a held disc into a stalled disc.
In Context
The mark is the difference between an unhurried thrower and a rushed one. A good mark slides with pivots, denies break attempts, and never reaches — they make the thrower do the work. See Sliding for the technique.
From an offensive perspective, the mark is the clock. Every second under a live mark is a second the rest of the field is recovering shape. This is why we play to release before the mark sets — see the 2 Second Window — and why every thrower must be comfortable using pivots and fakes to move the marker rather than wait for them to choose a side.
The phrase "break the mark" means throwing through the side the marker is closing — typically requiring an IO or roll-curve release.