Stories We Like: A Guide to the Comma
1610s, "anything spread out;" 1640s, "act of expanding," from French expansion, from Late Latin expansionem (nominative expansio) "a spreading out," noun of action from past participle stem of Latin expandere (see expand).
expansion ex·pan·sion (ĭk-spān'shən)
An increase in size.
The spreading out of a structure, such as a tendon.