acronym –

a word formed from the initial letters or groups of letters of words in a set phrase or series of words,

OPEC from Organization of Petroleum Exporting Countries, loran from long-range navigation.

antonym-

a word opposite in meaning to another.

Fast is an antonym of slow.

(An antonym is “A word having a meaning opposite to that of another word: The word wet is an antonym of the word dry.”)

Or you may be referring to an acrostic – “an arrangement of words or lines in which a series of initial, final, or other corresponding letters, when taken together, stand in a set order to form a word, a phrase, the alphabet, or the like. A famous acrostic was made on the Greek for Jesus Christ, God’s Son, Savior: Iesous Christos, Theou Uios, Soter (ch and th being each one letter in Greek). The initials spell ichthus, Greek for fish; hence the frequent use of the fish by early Christians as a symbol for Jesus. There are several alphabetic acrostics (pertaining to the Hebrew alphabet) in the Bible, e.g., in Ps. 119 and Lamentations Lamentations, book of the Bible, placed immediately after Jeremiah, to whose author it has been ascribed since ancient times. It was probably composed by several authors. It is a series of five poems mourning the destruction of Jerusalem by Babylon.” – definition from The Columbia Electronic Encyclopedia.

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Antonym is used to give the opposite word

For example the antonym of good is bad