Friction
Friction is the accumulated small cost of effort, time, decision and inconvenience standing between a person and an action. The working claim in behaviour design is that lowering those costs changes what people do more reliably than raising their motivation does. It is a useful description rather than a measured construct, and this page says so: there is no effect size for "reducing friction", and anybody quoting one has invented it. The evidence that is real sits in the adjacent literatures, under other names.
What it is
Friction is not one obstacle. It is several small ones stacked: the bag that is not packed, the class that has to be booked, the forty minutes of travel, the choice of what to do once you arrive.
Take two people with identical intentions to train. One has a gym eleven minutes from the flat, a kit bag packed the night before and a fixed session already written down. The other has a forty-minute journey, a bag to assemble and a decision to make about what to train on arrival. The second person is not less motivated. They are paying four small costs the first person has already paid, and they are paying them at the moment when the decision is weakest.
The term has no single originator in the research literature, which is part of why this page is tagged mixed. It circulates as practitioner vocabulary, and the vault created this note as the general concept sitting above two more specific ones it already held, elements of simplicity, which is Fogg's six-factor breakdown of what makes a behaviour easy, and cognitive ease, which is about processing fluency rather than the cost of acting.