What is it called when each word of the sentence starts with same letter?

There isn't a specific, widely-accepted term for when each word in a sentence starts with the same letter.

However, here are some terms you could use to describe it:

* Alliteration: This is the most common term for this type of wordplay, although it usually refers to the repetition of consonant sounds, not just the first letter.

* Initialism: This term refers to a word formed from the first letters of a phrase, but it can be extended to describe a sentence where all words start with the same letter.

* Letter repetition: This is a more general term that simply describes the repetition of the same letter.

* Wordplay: This is a broad term that encompasses various forms of playful language, including letter repetition.

You could also simply say "The sentence uses the same letter at the beginning of each word."

It's important to note that this type of sentence structure is often considered stylistically awkward and is more common in playful or humorous contexts.