Slurry
also: coffee slurry, the bed
The mixture of ground coffee suspended in water during brewing, before it drains or is separated.
Slurry is the working mixture of ground coffee and water that exists while a brew is happening: grounds suspended, swirling, or settling in the liquid before the coffee drains away or the grounds are separated out. In immersion brewing the whole steep is one long-lived slurry; in percolation it is the wet bed that water keeps flowing through.
Why the word is useful: a lot of brewing technique is really about managing the slurry. When you stir or swirl, you are moving the slurry to even out contact and speed extraction. When you watch the bloom bubble and rise, you are reading the slurry. When the slurry temperature drops during a long pour-over, extraction slows.
What to look for: a healthy slurry is evenly wetted with no dry clumps floating on top and no grounds caked dry against the walls. Fine particles, or fines, tend to migrate to the bottom of the slurry over time, which is part of why the last sips of a French press can taste muddier. Picture the slurry and many brewing problems become easy to see.