The coarseness and conversely fineness of the ground coffee beans.
Grind size range from very fine (e.g., for Turkish coffee, espresso) to medium (e.g., for drip), to coarse (e.g., for French Press or cold brew). It is generally designated on the subjective scale of “fine” to “coarse” or by the type of brew it will be used for (e.g., in a cafe or specialty bean order, you can often choose to grind for drip, French Press or your preferred method). However, it can also be measured objectively and specified by the size of the coffee particles on the micrometer scale, for example see Kruve’s “brewler”.
Finer grinds have more surface area exposed to the water, and smaller volume for the water to diffuse into each bean. Therefore, grinding finer will generally result in increase extraction compared to an identical brew with coarser grinds. However, grind size impacts other variables, so it is not always so simple to alter the grind and while keeping everything else the same (e.g., finder grinds will often take more time or force for water to percolate through, so grinding finder may also lengthen the contact time, which also increases extraction)