TDS (total dissolved solids)
also: total dissolved solids, strength
The percentage of dissolved coffee in your cup; the measurable definition of strength.
TDS stands for total dissolved solids: the share of your finished drink that is actually dissolved coffee rather than water. It is usually given as a percentage by weight, and it is the precise meaning of strength. A weak cup and a strong cup can be extracted equally well; they just hold different amounts of dissolved material.
Typical figures: filter coffee lands around 1.2 to 1.5 percent TDS, while espresso is far more concentrated at roughly 8 to 12 percent. The rest, around 98 to 99 percent for filter, is water.
You change TDS mainly through your brew ratio: more coffee per unit of water raises it, more water lowers it. That is different from extraction-yield, which measures how much you pulled out of the grounds. A drink can be strong but under-extracted, or weak but well extracted. Hobbyists measure TDS with a refractometer, then combine it with the brew weights to calculate extraction yield. Most drinkers just adjust ratio to taste.