An instance of spending the night as a guest at another's home. (noun)
An overnight guest. (noun)
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.
Use "sleepover" in a sentence
"(Next sleepover is in two weeks time with Pa and Nanna Pam while we go to a wedding.)"
"But the good thing about the sleepover is that the three of them just went off and did stuff."
"Wait a minute - how the heck do you define the word sleepover anyway?"