Definition: Agreeableness is one of the five basic elements, or traits, of personality according to the "Big Five" theory of personality. A person who is agreeable generally has positive social interactions, is pleasant to be around, works to help others, and cooperates well in group situations. He also tends to show affection readily and often. In other words, an agreeable individual is highly prosocial.
A person who is low in agreeableness shows the opposite behaviors, including being difficult and demanding in groups, being untrusting of others' intentions and lacking in social skills. Most people fall somewhere between the two extremes.
Agreeableness tends to increase gradually until adulthood. It's natural for kids and teens to go through periods of low agreeableness, such as during puberty. Even then, though, some tweens will be more agreeable than others when dealing with changes in their bodies and stresses in their environment.
Related terms: Neuroticism, introversion, extraversion, openness to experience, conscientiousness
Source:
Rathus, PhD, Spencer. Psychology: Concepts and Connections, Brief Version. 8th edition. 2007. Belmont, CA: Thomson, Wadsworth.