What is expansion? What is expansion? The act or process of expanding: the new nation's expansion westward. The state of being expanded.An expanded part: an expansion of a river. A product of expanding: a book that is an expansion of the author's Ph.D. thesis. The extent or amount by which something has expanded.Mathematics A quantity written in an extended form, such as in a sum or product of terms.Mathematics The process of obtaining this form.An expanse.A period of increased economic or business activity.