The disc of spent grounds left in the brewer at the end of a brew. The word comes from espresso — where the compacted, wet hockey-puck-shaped cake of grounds is the literal physical object you knock out of the portafilter — but it's now used for filter coffee too.
Reading your puck is one of the cleanest free diagnostic tools you have. The shape, surface, and texture of the spent bed tell you what happened during the brew with no equipment required.
Pour-over puck shapes and what they mean
Flat, slightly concave (centre very slightly lower than rim). The ideal. Even pours, even bed, even extraction. This is the shape Scott Rao's "spin" was designed to produce.
Doughnut (high centre, low rim). The bed channeled in the middle. Water cut down through the centre and avoided the outer ring. Grind finer or pour more inward.
Crater (low centre, high rim). Pour was too forceful in the middle, blew a hole through the bed. The outer ring under-extracted. Slower, gentler pours next time.
Sloped (one side higher than the other). The brewer wasn't level, or you poured asymmetrically. Boring fix: brew on a level surface.
Espresso puck reading
For espresso the puck should come out as a single intact disc, slightly damp on top but firm. Crumbly, soupy, or holey pucks indicate distribution or extraction problems. The colour should be uniform brown — light spots mean dry channels, dark spots mean wet ones.
Take ten seconds at the end of every brew to look at the puck. It costs nothing and over weeks of brews it teaches you more than any TDS reading will.