TO ASSERT -- To state an opinion, claim a right, or establish authority. If you assert yourself, you behave in a way that expresses your confidence, importance or power and earns you respect from others. - From the Oxford English Dictionary
Assertiveness is standing up for your right to be treated fairly. It is expressing your opinions, needs, and feelings, without ignoring or hurting the opinions, needs, and feelings of others.
Because people want to be liked and thought of as 'nice' or 'easy to get along with', they often keep their opinions to themselves, especially if those opinions conflict with other people's. But this sometimes leads to being taken advantage of by people who are not as nice or considerate. Asserting yourself will stop others from cheating you and you from cheating yourself out of what you deserve.
Assertive behavior includes: