A stutter or stammer was an occurrence in which an individual's speech became halting or repetitive. It could be brought on by being nervous or occur as part of a neurological or technological problem.
In 2365, when a partially malfunctioning Data responded to Captain Jean-Luc Picard's question about overriding doors in a facility on Iconia by saying "Blue, blue, blue.", Picard replied, "I hope that's not a stutter." (TNG: "Contagion")
A revived and Kobali-ized Lyndsay Ballard, upon being told by Harry Kim in 2376 that he'd delivered her eulogy, responded by saying, "I just hope you didn't stutter too much," due in part because she felt he was "a terrible public speaker." (VOY: "Ashes to Ashes")
Reginald Barclay stammered when he was lying, something he hadn't done in years prior to The Doctor asking him where Admiral Kathryn Janeway was following her unexplained absence in 2404. Recognizing Barclay's tell, The Doctor eventually convinced him to tell what he knew. (VOY: "Endgame")