An acrostic is a poem or other word composition in which the first letter (or syllable, or word) of each new line (or paragraph, or other recurring feature in the text) spells out a word, message or the alphabet. The term comes from the French acrostiche from post-classical Latin acrostichis, from Koine Greek ἀκροστιχίς, from Ancient Greek ἄκρος "highest, topmost" and στίχος "verse". As a form of constrained writing, an acrostic can be used as a mnemonic device to aid memory retrieval. When the last letter of each new line (or other recurring feature) forms a word it is called a telestich (or telestic); the combination of an acrostic and a telestich in the same composition is called a double acrostic (e.g. the first-century Latin Sator Square).

Acrostics are common in medieval literature, where they usually serve to highlight the name of the poet or his patron, or to make a prayer to a saint. They are most frequent in verse works but can also appear in prose. The Middle High German poet Rudolf von Ems for example opens all his great works with an acrostic of his name, and his world chronicle marks the beginning of each age with an acrostic of the key figure (Moses, David, etc.). In chronicles, acrostics are common in German and English but rare in other languages.

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. These acrostics occur in the Hebrew Bible in the first four of the five chapters of the Book of Lamentations, in the praise of the good wife in Proverbs 31:10-31, and in Psalms 9-10, 25, 34, 37, 111, 112, 119 and 145. Notable among the acrostic Psalms is the long Psalm 119, which typically is printed in subsections named after the 22 letters of the Hebrew alphabet, each section consisting of 8 verses, each of which begins with the same letter of the alphabet and the entire psalm consisting of 22 x 8 = 176 verses; and Psalm 145, which is recited three times a day in the Jewish services. Some acrostic psalms are technically imperfect. For example, Psalm 9 and Psalm 10 appear to constitute a single acrostic psalm together, but the length assigned to each letter is unequal and five of the 22 letters of the Hebrew alphabet are not represented and the sequence of two letters is reversed. In Psalm 25 one Hebrew letter is not represented, the following letter (Resh) repeated. In Psalm 34 the current final verse, 23, does fit verse 22 in content, but adds an additional line to the poem. In Psalms 37 and 111 the numbering of verses and the division into lines are interfering with each other; as a result in Psalm 37, for the letters Daleth and Kaph there is only one verse, and the letter Ayin is not represented. Psalm 111 and 112 have 22 lines, but 10 verses. Psalm 145 does not represent the letter Nun, having 21 one verses, but one Qumran manuscript of this Psalm does have that missing line, which agrees with the Septuagint. Some, like O Palmer Robertson, see the acrostic Psalms of book 1 and book 5 of Psalms as teaching and memory devices as well as transitions between subjects in the structure of the Psalms.

Often the ease of detectability of an acrostic can depend on the intention of its creator. In some cases an author may desire an acrostic to have a better chance of being perceived by an observant reader, such as the acrostic contained in the Hypnerotomachia Poliphili (where the key capital letters are decorated with ornate embellishments). However, acrostics may also be used as a form of steganography, where the author seeks to conceal the message rather than proclaim it. This might be achieved by making the key letters uniform in appearance with the surrounding text, or by aligning the words in such a way that the relationship between the key letters is less obvious. These are referred to as null ciphers in steganography, using the first letter of each word to form a hidden message in an otherwise innocuous text. Using letters to hide a message, as in acrostic ciphers, was popular during the Renaissance, and could employ various methods of enciphering, such as selecting other letters than initials based on a repeating pattern (equidistant letter sequences), or even concealing the message by starting at the end of the text and working backwards.

A well-known acrostic in Greek is for the phrase JESUS CHRIST, GOD’S SON, SAVIOUR, the initial letters of which spell ΙΧΘΥΣ (ICHTHYS), which means fish:

According to Cicero, acrostics were a regular feature of Sibylline prophecies (which were written in Greek hexameters. The type of acrostic is that known as a “gamma acrostic” (from the shape of the Greek letter Γ), where the same words are found both horizontally and vertically. Cicero refers to an acrostic in this passage using the Greek word ἀκροστιχίς.

The 3rd-century BC didactic poet Aratus, who was much admired and imitated by Cicero, Virgil and other Latin writers, appears to have started a fashion for using acrostics. One example is the famous passage in Phaenomena 783–7 where the word λεπτή 'slender, subtle' occurs as a gamma acrostic and also twice in the text, as well as diagonally in the text and even cryptically taking the initial letters of certain words in lines 2 and 1:

Several acrostics have recently been discovered in Roman poets, especially in Virgil. Among others, in Eclogue 9 the acrostic VNDIS 'in the waves' (lines 34–38) immediately precedes the words quis est nam ludus in undis? 'for what is your game in the waves?'', and DEA DIO (i.e. dea Dione 'the goddess Dione') (lines 46–51) in a passage which mentions the goddess Dione (another name for Venus). In Eclogue 8, alongside a passage dedicating the poem to an unnamed person and asking him to accept it, Neil Adkin reads the words TV SI ES ACI (i.e. accipe) ('if you are the one, accept!').