Real Dictionary

stammer
verb: If you stammer, you speak with difficulty, hesitating and repeating words or sounds.

stammer in American English
act, instance, or habit of stammering
to speak or say with involuntary pauses or blocks, often with rapid repetitions of syllables or initial sounds, as temporarily from excitement, embarrassment, etc. or chronically as a result of muscle spasms believed to result from mental conflicts; stutt
intransitive verb: to speak with involuntary breaks and pauses, or with spasmodic repetitions of syllables or sounds
noun: a stammering mode of utterance
transitive verb: to say with a stammer

stammer in British English
noun: a speech disorder characterized by involuntary repetitions and hesitations
verb: to speak or say (something) in a hesitant way, esp as a result of a speech disorder or through fear, stress, etc