あいがみ　（合紙）　slip sheet
（1）印刷用語では、印刷したインキの生かわきのために生ずる裏移りを防ぐために、印刷ずみの紙の間に入れる紙。（2）製本用語では、見返し紙に和紙を使ってノリ入れをした場合、見返し紙の間が密着しないために入れる紙。→間紙
http://72.14.207.104/search?q=cache:zpBKUWLedPEJ:www.if-j.co...

I also found "inserting paper" for 合紙 on Glova, but had a lot of trouble sifting through irrelevant Google hits to find any supporting evidence of that...

However, I also found the following (which uses the same Japanese definitions you supplied)
■合紙（あいし）【laminated board, lined board, laminated paper】
　数量の確認を容易にするなどの目的で、印刷物の間に挟み込まれた紙。又は紙を挟むこと。印刷や製本の時に、裏うつりなどを防ぐ目的で間に挟む紙。又は紙を挟むこと。「間紙（かんし）【interleaf】」とも言う。※漢字表記の場合には、「合紙（ごうし）【board lamination】」と紛らわしく、「間紙（かんし【interleaf】」の方が間違いは少ない。
http://72.14.207.104/search?q=cache:yqud4YNjkYAJ:www.konet.c...

Finally, http://www.google.com/search?sourceid=navclient&ie=UTF-8&rls...
Definitions of "slip sheet" on the Web:

Tissue between stacked proofs to prevent offseting of ink to next print.
www.clt.astate.edu/elind/prtmkgvoc.htm

A flat sheet of material used as a base upon which goods and materials may be assembled, stored and transported.
www.actionpackaging.com/html/glossary.htm

Slip Sheets are thin pallet sized sheets made of plastic or fibreboard. Goods are placed on the slip sheets and then secured - usually by pallet shrouds or pallet wrap. A specially adapted fork lift truck drags the load onto itself and places it into a truck. At the destination the process is reversed. Slip sheets are very common in the United States but less so in Europe where there is no common standard.
en.wikipedia.org/wiki/Slip_Sheet

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Note added at 29 mins (2006-05-01 20:46:03 GMT)
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Sorry - the very 1st ref. I supplied above also had the following, which I must have chopped off...
"あいし、あいがみ （間紙、合紙） interleaving paper/slip sheet/smut sheet/set orr sheet"

Possibly "interleaf" or "interleaving paper" （間紙） are other suitable alternatives.