Real Dictionary

tongue twister in American English
a phrase or sentence that is hard to say fast, usually because of alliteration or a sequence of nearly similar sounds (Ex.: six sick sheiks)
noun: a word or sequence of words difficult to pronounce, esp. rapidly, because of alliteration or a slight variation of consonant sounds, as “She sells seashells by the seashore”

tongue twister in British English
noun: a sentence or phrase that is difficult to articulate clearly and quickly, such as Peter Piper picked a peck of pickled pepper

tongue-twister
countable noun: A tongue-twister is a sentence or expression which is very difficult to say properly, especially when you try to say it quickly. An example of a tongue-twister is 'Red leather, yellow leather'.