Definition: Conscientiousness is one of the five basic elements of personality, which are called the "Big Five" by psychologists.
Someone who is high in conscientiousness tends to be on time or early for appointments, is highly responsible and works toward long-term goals with little or no supervision. A person who is low in conscientiousness may not come through on obligations, tends to complete assignments hastily or not at all and often makes impulsive decisions. There is, of course, a range of behaviors between these two extremes.
Rathus, Spencer. Psychology: Concepts and Connections, Brief Version. 8th edition. 2007. Belmont, CA: Thomson, Wadsworth.