When someone speaks a language well, we often say that person is "fluent." "Fluent" is derived from the Latin word "fluere," which means "to flow." So, if speech is fluent, it flows uninterrupted.
A person who stutters experiences "disfluencies," or interruptions in the flow of speech. Of course, most people experience speech disfluencies to some degree or another — that's why we pause and say "um," "like" or various other verbal crutches. But for some people, these disfluencies are far more significant and can't be bridged by a word or phrase.
Disfluencies can take many different forms, including longer than normal pauses in speech; repeated words, phrases or letters; and most severely, complete blockage of speech. Often, people with stutters don't experience disfluencies all the time. Rather, certain situations (especially public speaking) can trigger stuttering. Many find various ways to disguise or hide their disfluencies with strategies that include rearranging the order of sentences, claiming to forget what they were saying or simply remaining mute. A stutter can also affect a person's choices in life, including profession. A person with a stutter might, for instance, decide against a life in politics and instead find work that requires little talking.
Roughly 1 percent of the global adult population stutters and, for reasons that remain unknown, men who stutter outnumber women 5 to 1 [sources: The Stuttering Foundation, NIDCD]. While some forms of stuttering can accompany a syndrome such as Parkinson's disease, and in some cases people acquire a stutter due to head trauma, the most common type of stuttering by far is developmental.
Typically, children who stutter begin to develop disfluencies between ages 2 and 4. About 5 percent of children stutter. Of that number, most (75 percent) will lose their disfluencies as they mature, but for some (the remaining 25 percent) the condition never really goes away. For the latter group, stuttering may have become a source of anxiety and negative self-image, making the problem continue as the disfluencies get locked into the developing brain, rather than fade away [sources: The Stuttering Foundation, NIDCD].
Recent research suggests that there might be genetic traits that predispose some people to stutter. If so, it appears that life experience is critical to triggering and reinforcing disfluencies.