1 [uncountable and countable]
when something increases in size, range, amount etc [= growth; ↪ expand]
an expansion in student numbers
2 [uncountable and countable]
when a company, business etc becomes larger by opening new shops, factories etc [↪ expand]:
The industry underwent a period of rapid expansion.
an idea, story etc that is based on one that is simpler or more general:
The novel is an expansion of a short story he wrote about forty years ago.