A stammer is a disorder of speech. There are stoppages and disruptions which interrupt the smooth flow and timing of the speech. These stoppages may take the form of repetitions of sounds, syllables or words - like saying da-da-daddy. There may also be prolonged sounds - so that words seem to be stretched out - like saying mmmmmummy. It can also involve silent blocking of the airflow of speech, so that no sound is heard. As a result, the speech may sound forced, tense or jerky.
This is a companion discussion topic for the original article at https://patient.info/brain-nerves/stammer-stutter