General terms:
* Acrostic: This is the most common term for a poem where the first letter of each line spells out a word or phrase.
* Alphabet poem: This type of poem uses the letters of the alphabet in sequence, often with each letter serving as the starting letter for a new line.
* Initial poem: This is a general term for a poem that uses initial letters to create a pattern or message.
More specific terms:
* Clerihew: This is a four-line poem where the first two lines rhyme and the last two lines rhyme. The first line is usually a person's name.
* Tanka: This is a Japanese form of poetry with five lines and a specific syllable count: five, seven, five, seven, seven. The first line is often an acrostic.
Other possibilities:
* Concrete poetry: This form of poetry uses visual elements and the arrangement of letters to create a specific image or message.
* Pattern poetry: This term encompasses any poem where the arrangement of the letters or lines creates a visual pattern or design.
Ultimately, the best term to use will depend on the specific form and structure of the letter poem.