Acrostic comes from the Greek, 'akrostichis'. It means a rhyme in which the first letter of each line composes a word or sentence vertically from top to bottom.
Acrostic poetry usually talks about what is the arrangement of letters that make up a sentence at the beginning of a line.
To be more clear, see the meaning and example of acrostic poetry below!
Definition of Acrostic Poetry
Acrostic poetry is a rhyme or poem in which the first letter of each line composes a word or sentence vertically from top to bottom.
 Acrostic poetry uses letters in a word to start each line in the poem, all lines in the poem tell or describe an important word topic. The rhyme pattern and number of lines in acrostic poetry can vary because acrostic poetry is more than descriptive poetry that explains the formed words.
Examples of Acrostic Poetry
                  SKY
Silently the cloud crosses the light blue expanse.
Keeping the earth under its warm light.
You don't need to feel alone because the sky is always with you.