Final answer:
Temperament is the innate pattern of arousal and mood evident since birth, with a biological basis, shaped by reactivity and self-regulation as well as environmental factors.
Step-by-step explanation:
The concept in question is temperament, which refers to a person's general pattern of attention, arousal, and mood that is evident at birth. Contemporary psychologists consider temperament to have a biological basis, as it manifests early in life. Thomas and Chess's research categorized babies into three temperaments: easy, difficult, or slow to warm up. Though innate, environmental factors like family interactions and maturation can influence how children's personalities eventually present themselves.
Furthermore, it's important to understand that temperament is one of the key factors in early development that shapes future personality and behavior. Reactivity and self-regulation are two dimensions of temperament that play significant roles in adult personality. The former refers to our response to new or challenging stimuli, while the latter concerns our ability to control that response.