Attrition is an effect that causes a fraction of all units and most resources to disappear each watch. It represents units breaking down and resources decaying. Generally speaking, low-TL units have higher attrition than higher-TL units and cheap units have higher attrition than expensive units.
Attrition is measured in half-life, which is different for every unit and resource. A high half-life represents low attrition.
Uniquely, hexacarbide does not undergo attrition.