Definition: A trait is an element of personality that is relatively stable throughout the lifespan and across contexts. In other words, a trait is a consistent element of self that makes that person who he is. Traits are the general tendencies you'd list if you were asked to describe your child or your best friend.
Traits affect our thoughts, emotions and behaviors. Psychologists who support the trait theory of personality believe that there are five basic traits (called the "Big Five"): extraversion, openness to experience, conscientiousness, neuroticism and agreeableness.
McAdams, Dan, & Olson, Bradley. Personality Development: Continuity and Change Over the Life Course. Annual Review of Psychology. 2010. 61: 517-542.