Connotation
A stubborn person may be described as being either strong-willed or pig-headed. Although these have the same literal meaning (i.e. stubborn), strong-willed connotes admiration for the level of someone's will, while pig-headed connotes frustration in dealing with someone. Likewise, used car and previously owned car have the same literal meaning, but many dealerships prefer the latter, since it is thought to have fewer negative connotations.