A mnemonic can be many things but very often forms into a quasi-acronym or contraction style of abbreviation. Note the following from http://www.techterms.com/definition/mnemonic …

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A mnemonic (pronounced “nemonic”) is a pattern that can be used as an aid for memorizing information. Most often, this pattern consists of letters or words. For example, the phrase “Every Good Boy Does Fine” can be used to help music students remember the notes of the staff, E, G, B, D, and F. The name “Roy G. Biv” is often used to memorize the order of colors in a rainbow (or other light spectrum) — Red, Orange, Yellow, Green, Blue, Indigo, Violet.

While initials of words are commonly used as mnemonic devices, rhyming words and poems can also be used to memorize information. Furthermore, images can be associated with words or phrases to help memorize them. Because the human brain organizes information in “chunks,” mnemonics help people categorize information better, which makes it easier to remember.
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Also see http://en.wikipedia.org/wiki/Mnemonic