What is an 'acrostic poem'?

An acrostic poem is one that uses a Word or phrase (usually the theme or the underlying subject matter of the poem) written vertically. Each letter of the Word/phrase then acts as the beginning letter for a new line of the poem. Whatever is written using each letter must connect to the subject matter.

For example if the subject matter is CATS, then the poem could be:

Cute and cuddly

Always up to mischief

Time is always spent playing

Stupendous fun

Or if the subject matter is DOGS, then the poem could be:

Dashing, daring, delightful

Oh he's under the table

Growling is a bad thing

Shh go to bed