When identifying a "bully" we include behaviors such as:

• Calling other people names
• Using physical violence
• Writing mean, inappropriate things about other people
• Taking items that belong to other people
• Saying rude things to other people or acting out rude behaviors
• Generally making other people feel uncomfortable

Being a "bully" is much more that teasing or showing one or more of these behaviors occasionally.  A "bully" displays the behaviors described above over a long period of time.