Is sadism a personality trait or just meanness?

No simple answer. Sadistic personality is not in the dsm (the diagnostic manual of mental disorders). However, it is recognized in the clinical literature and does exist, whether or not it is an "official" diagnosis. Sadism can be expressed in many ways (not just sexual, contrary to the other doc's comment). Whether it is a personality syndrome or just a "mean streak" depends on how extreme and pervasive.
Disorder. The dsm classifies sadism as a disorder. It is when a person has acted on sexual urges with a nonconsenting person to cause psychological or physical suffering and the person acting on their urges is sexually excited.