A measurement of the available water inside a coffee bean — water free enough to participate in chemical reactions, microbial growth, and aroma loss. Written Aw, on a scale from 0 to 1 where 1 is pure liquid water and 0 is bone dry.
Roasted coffee typically sits between 0.20 and 0.40 Aw. Below 0.20 the bean has lost its expressiveness and is approaching desiccation; above 0.40 it's at risk of mould and aroma degradation.
Why it's not the same as moisture content
Total moisture content tells you how much water is in the bean. Water activity tells you how much of it is active — chemically available rather than locked into the bean's structure. Two beans with identical moisture readings can have different Aw, and the high-Aw one will stale faster, mould-risk faster, and lose aromatics faster regardless of how much total water it contains.
Why it matters in practice
Specialty roasters monitor Aw because it's the strongest predictor of how a coffee will change in storage. A freshly roasted bean at 0.30 Aw stored at 0.50 Aw equilibrium humidity will gain water until it matches its environment, accelerating staling. Storage in low-RH environments (or sealed bags with humidity-control packs) preserves Aw and keeps the bean tasting like itself for longer.
For green coffee, 0.55–0.65 Aw is the standard storage window. Above 0.70, mould and ochratoxin-A become risks; below 0.45, the green has been over-dried and will roast unevenly.
You can measure Aw with a benchtop water-activity meter (Rotronic, Aqualab). Most home enthusiasts will never see one — the relevance for the drinker is conceptual: it's why sealed bags with one-way valves exist.