Acting or done without or against one's will: an involuntary participant in what turned out to be an argument.
Not subject to control of the volition: gave an involuntary start. See Synonyms at spontaneous.
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.