A pattern of observable behaviors that is the expression of a subjectively experienced feeling state (emotion).
Examples of affect include sadness, elation, and anger.
In contrast to mood, which refers to a pervasive and sustained emotional "climate," affect refers to more fluctuating changes in emotional "weather." What is considered the normal range of the expression of affect varies considerably, both within and among different cultures.

American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders. 5th edition. Washington, DC. American Psychiatric Association. 2013. Page 817.