A trait is a characteristic, a capacity, an ability. It is much more refined and narrowed, less abstract, close to what is
has-a instead of naive and crude
This is very important. Traits are compostable (mixable), while classes are not. This is a fundamental and principal difference.
In the very first version of the OO DSL for Zeta Lisp (a precursor of Common Lisp) has flavours which were just another name for traits and suggests that they could be mixed (composed).
See also Structures which is a very related concept.