Assertiveness is the quality of being self-assured and confident without being aggressive. It is a learnable skill and mode of communication and it is often linked to self-esteem. High scorers on this scale are dominant, forceful, and socially ascendant. They speak without hesitation and often become group leaders. Low scorers prefer to keep in the background and let others do the talking.
Sources: Wikipedia, Costa, P. T. and McCrae, R. R. (1992). NEO PI-R Professional Manual. Psychological Assessment Resources, Inc., Odessa. FL