The word "affect" generally refers to the influence or impact that one thing has on another. In psychological terms, it can also denote the expression of emotions or feelings. For instance, when a teacher's encouraging words positively influence a student's motivation, we say that the teacher affects the student.
The word "affect" comes from the Latin "afficere," meaning "to do something to," "to have an influence on." It has evolved through Middle English and Old French before settling into modern usage in English.