To speak or say something, especially the first part of a word, with difficulty, for example pausing before it or repeating it several times. (Cambridge Dictionary)
To speak with involuntary disruption or blocking of speech (as by repetition or prolongation of vocal sounds) (Merriam – Webster)
To move or act in a halting or spasmodic manner. (Merriam – Webster)
Why we stutter?
How to stop or reduce a stutter.