"Respecting the rights of yourself and others equally." This is the definition of assertion we use at OCS.
While many of us consider this to be a basic skill of management and leadership, I find it interesting that the need of managers and leaders for development in this skill is still widespread. Equally interesting to me is what I find to be the incomplete understanding of the principle of assertion.
The approach taken at OCS is that the skill of assertion is underpinned by the principle of rights. Understanding the principle of rights is more important than describing behaviours such as assertive, passive or aggressive. We need to have the context of what is happening with people's rights, to be able to decide whether a particular behaviour is passive, aggressive or assertive.
What may seem passive behaviour on the face of it (e.g. not speaking), may in certain circumstances of people rights, be aggressive. Consider the withholding of information to which a person is entitled, or the exclusion of someone from a social circle. This is sometimes referred to as passive aggressive behaviour. What superficially seems to be aggressive behaviour (shouting) may be assertive. Consider dealing with a home intruder for example. This all becomes very clear if this is viewed through the lens of the rights of the people involved.
Industrially, the issue of bullying becomes clearer when the context of rights is understood and used in assessing the situation. I regularly have managers say to me they are not "allowed" to raise an under performance issue with an employee as the person will accuse them of bullying. The person may accuse a manager of bullying, but if the employee is not meeting their obligation of performing appropriately and respecting the organisation's and their manager's right to have people perform to a standard, then the manager has every right to raise the issue. In terms of rights, there are rights the employee has that the manager needs to respect to handle this interaction assertively and not aggressively. This is another subject in its own right (pun intended) so I am happy to discuss this in more depth if you want to contact me.