[ex.pan.sion] n (1611) 1: expanse
2: the act or process of expanding "territorial ~"
3: the quality or state of being expanded
4: the increase in volume of working fluid (as steam) in an engine cylinder after cutoff or in an internal combustion engine after explosion
5. a: an expanded part b: something that results from an act of expanding "the book is an ~ of a lecture series"
6: the result of carrying out an indicated mathematical operation: the expression of a function in the form of a series -- ex.pan.sion.al adj