Personality concerns the most important, most noticeable parts of an individual's psychological life. Personality concerns whether a person is happy or sad, energetic or apathetic, smart or dull. Over the years, many different definitions have been proposed for personality. Most of the definitions refer to a mental system -- a collection of psychological parts including motives, emotions, and thoughts. The definitions vary a bit as to what those parts might be, but they come down to the idea that personality involves a pattern or global operation of mental systems. Here are some definitions:
"Personality is the entire mental organization of a human being at any stage of his development. It embraces every phase of human character: intellect, temperament, skill, morality, and every attitude that has beeen built up in the course of one's life." (Warren & Carmichael, 1930, p. 333)
(In an acknowledged overstatement...) "Personality is the essence of a human being." (Hall & Lindzey, 1957, p. 9, characterizing statements by Gordon Allport)
"An individual's pattern of psychological processes arising from motives, feelings, thoughts, and other major areas of psychological function. Personality is expressed through its influences on the body, in conscious mental life, and through the individual's social behavior." (Mayer, 2005)
References
Hall, C. S., & Lindzey, G. (1957). Theories of personality. New York, NY: John Wiley & Sons.
Mayer, J. D. (2005). A classification of DSM-IV-TR mental disorders according to their relation to the personality system. In J. C. Thomas & D. L. Segal (Eds.), Comprehensive handbook of personality and psychopathology (CHOPP) Vol. 1: Personality and everyday functioning. New York, NY: John Wiley & Sons.
Warren, H. C., & Carmichael, L., Elements of human psychology (Rev. Ed.; Boston, MA: Houghton Mifflin, 1930), p. 333/Cited in Allport, Pattern & growth in personality (1937/1961, p.36).