1. A state of arrested development in an adult, characterized by retention of infantile mentality, accompanied by stunted growth and sexual immaturity, and often by dwarfism.
2. 1. Marked immaturity, as in behavior or character: "infantilism, which is the subtext of so much American cinema and culture" (John Simon).
   2. An infantile act or remark.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.