Is it ensure, insure, or assure?
1843, from the adjective homophone (1620s), from Greek homos "same" (see homo- (1)) + phone "sound" (see fame (n.)). Related: Homophonic.
Two words that sound alike. This category includes words that are spelled the same, such as trunk (of an elephant) and trunk (a storage chest), as well as words spelled differently, such as deer and dear.