Shapiro: "Stuttering refers to individualized and involuntary interruptions in the forward flow of speech and learned reactions thereto interacting with and generating associated thoughts and feelings about one's speech, oneself as a communicator, and the communicative world in which we live."
Hood: "Stuttering is a communication disorder characterized by excessive involuntary disruptions in the smooth and rhythmic flow of speech, particularly when such disruptions consist of repetitions or prolongations of a sound or syllable, and when they are accompanied by emotions such as fear and anxiety, and behaviors such as avoidance and struggle."
Cooper and Cooper: Stuttering is a clinical syndrome characterized by abnormal and persistent disfluencies in speech, accompanied by affective, behavioral and cognitive patterns. (the A, B, C's of stuttering)
Quesal: Stuttering is a disorder of fluency characterized by various behaviors that interfere with the forward flow of speech. While all individuals are disfluent to some extent, on the surface what differentiates stutterers from nonstutterers is the frequency of their disfluency and/or the severity of their disfluency. However, the other factor that differentiates stutterers from nonstutterers is that almost invariably the disfluencies that the stutterer regards as "stutters" are accompanied by a feeling of loss of control. It is this loss of control, which can't be observed or experienced by the listener, that is most problematic for the stutterer. (from What is stuttering: a quick definition by Bob Quesal.
Jackson, Quesal, and Yaruss: "Stuttering is a neurobiological lack of integration of the underlying processes of planning and producing language and speech that, upon verbal execution, can lead to interruptions in the acoustic speech signal (e.g., blocks, part-word repetitions, disfluencies) and physical struggle (e.g., tension). These surface behaviors may not be present, however, when the speaker exhibits communicative avoidance (e.g., circumlocutions, fillers). The underlying features may lead to surface behaviors, as well as emotional and cognitive reactions. Depending on the individual, these may result in significant difficulties in communication and an adverse impact on the speaker's quality of life. The physical symptoms, emotional and cognitive reactions, and impact on the speaker's life all comprise the disorder of stuttering." (ISAD 2012 What is Stuttering: Revisited
St. Louis: "Stuttering results from involuntary neuromotor breakdowns affecting the coordination of respiration, phonation, and articulation of speech. It typically-though not always--is (1) experienced by the speaker as a loss of voluntary control in saying words; (2) manifested as excessive and/or abnormal sound/syllable repetitions, prolongations, audible or silent blocks, or attempts to avoid these behaviors; and (3) associated with or triggered by variable amounts of psychic stress and negative emotion." St. Louis, K. O. (1997). What is stuttering: Some more current additions. In S. B. Hood, (Ed). Stuttering Words, 3rd ed., p.12. Memphis TN: Stuttering Foundation of America. (Invited definition.)
