An acrostic is a poem

Poetry

Poetry is a form of literary art in which language is used for its aesthetic and evocative qualities in addition to, or in lieu of, its apparent meaning...

 or other form of writing in which the first letter, syllable or word of each line, paragraph

Paragraph

A paragraph is a self-contained unit of a discourse in writing dealing with a particular point or idea. A paragraph consists of one or more sentences. The start of a paragraph is indicated by beginning on a new line. Sometimes the first line is indented...

 or other recurring feature in the text spells out a word or a message. As a form of constrained writing

Constrained writing

Constrained writing is a literary technique in which the writer is bound by some condition that forbids certain things or imposes a pattern.Constraints are very common in poetry, which often requires the writer to use a particular verse form....

, an acrostic can be used as a mnemonic

Mnemonic

A mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...

 device to aid memory retrieval. A famous acrostic was made in Greek for the acclamation JESUS CHRIST, GOD'S SON, SAVIOUR . The initials spell ICHTHYS

Ichthys

Ichthys, from Koine Greek: , is the Greek word for "fish"....

 (ΙΧΘΥΣ), Greek for fish – hence the frequent use of the fish as a symbol for Jesus Christ from the early days of Christianity to the present time.
Relatively simple acrostics may merely spell out the letters of the alphabet in order; such an acrostic may be called an 'alphabetical acrostic' or Abecedarius

Abecedarius

An abecedarius is an acrostic in which the first letter of every word, strophe or verse follows the order of the alphabet. Abecedarius is also a generic term for an alphabet book, which dates back to Biblical writings such as the Psalms, which used successive letters of the Hebrew alphabet as the...

.