Temperament refers to genetically influenced behavioral type; similar to a trait, but more suited to a categorical approach and more explicitly biological.
Other definition:
Temperament refers to the child's innate reactivity and self-regulation with respect to the domains of emotions, activity level, and attention. It is the child's organized style of behavior that appears early in development, such as fussiness or fearfulness that shapes the child's approach to his or her environment and vice versa.
Other /More definition:
Temperament refers to the innate characteristics that determine an individual"s sensitivity to various experiences and responsiveness to patterns of social interaction.

Moreover, Temperament refers to the child"s innate reactivity and self-regulation with respect to the domains of emotions, activity level, and attention; the child"s organized style of behavior that appears early in development, such as fussiness or fearfulness, which shapes the child"s approach to his or her environment and vice versa.
Other /More definition:
Temperament refers to a person"s characteristic modes of responding emotionally and behaviorally to environmental events, including such attributes as activity level, irritability, fearful distress, fearfulness, sociability and and positive affect.

Individual differences in styles of reaction that are present early in life; a consistent style or pattern of behavior.
Other /More definition:
Temperament refers to the physical core of personality, including emotional and perceptual sensitivity, energy levels, typical mood, and so forth; it is an individual"s base level of emotionality and reactivity to stimulation which, to a large extent, is genetically determined.