A poem in which the first letter of each line, read downward, spells out a word or message is called

An acrostic is a poem or other composition in which the first letters of each line form a word, phrase, or message when read vertically. An elegy is a mournful poem; a sonnet is a 14-line poem with a specific rhyme scheme; an ode is a lyric poem of praise. None of these other forms have the defining structural feature of spelling a word through initial letters.