7" / 10" / 12": The diameter of vinyl records.  7" items are usually singles that play at 45 rpm, 10" items could be a single or an EP, and 12" items could be a single (45 rpm) or an album (33 1/3 rpm).
Aftermarket: Click here.
b/w: "Bundled with" or "backed with."  It's a reference to the b-sides on singles (e.g. "Smells Like Teen Spirit" b/w "Even In His Youth").
Bootleg: Click here.
Box Set: A set of items that are packaged together.
COMPAC PLUS: A CD package similar to an ECO PAK.
Counterfeit: Click here.
Die-cut: A hole cut into the front of sleeves (forming a sort of "window") that allows the item to be partially seen.
Digipak: A thick paper (almost cardboard-like) CD package.  The paper is a folded one-piece section.  The CD rests in a normal plastic tray, like those in jewel cases.  The tray is glued to the paper.
ECO PAK FLP: Similar to a digipak, but features a small, plastic flap that closes the packaging.  When opened, the front cover folds out.
EP: Extended Play.
Flexi: An extremely thin piece of vinyl that is usually square.
Gatefold: A sleeve folded in the middle that opens like a book.  Most of the time such a sleeve holds two records or CDs.
IFPI: International Federation of the Phonographic Industry.
ISBN: International Standard Book Number.
Jewel Case: A plastic container for compact discs measuring 5" x 5" x .5".  A standard CD case.
Long-box: An oversized cardboard package that CDs used to be sold in.
LP: Long play.
Matrix: An engraving or stamped code.  For vinyl items, this describes the code etched into the run-out grooves on each side of the record.  For CDs, the code can be found on the metallic ring in the center of the disc.  (Many CDs may also have an IFPI code printed on the inner plastic ring.)
N/A: Not applicable.
Obi Strip: A piece of paper that typically comes with Asian releases.  (I think it's simply to inform Japanese consumers what the item is, in case they can't read English text.)  The paper's size will vary from item to item; some obi strips are folded and cover the edge of a CD case plus a small portion of the cases' front and back, others will wrap around the entire front and back of a jewel case.
Piracy / Pirate: Click here.
Re-issue: Click here.
Re-press: Click here.
Re-release: Click here.
RIAA: Recording Industry Association of America.
RPM: Revolutions per minute.  The speed at which records should be played.
Slimline Case: A plastic container for compact discs measuring 5" x 5" x .25".  Because the CD snaps onto the inside of the back of the case, there is no back insert (inlay).
Snappack: A folded sleeve.  See Come As You Are.  (This may also be spelled "snappak.")
White Label Promo: A promotional record that either has blank white labels or white labels with minimal text (artist's name, song titles, etc.).

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