The occurrence of the same letter or sound at the beginning of adjacent or closely connected words. For example: Short sharp shock from a Pink Floyd song, or "You'll never put a better bit of butter on your knife." (advertising slogan for Country Life butter)
It's a literary device where there is a string of words all starting with the same consonant. Here is an example: pretentious politicians pandering to the perilous preconceived populist perceptions. Not every word starts with a p but most of them do. Here is another example: Crazy Carol crashed her cousins Cadillac 'cause she was carelessly careening over the crest of the hill close to the canyon.