A word for really thin book pages

I have recently got a book which is almost 700 pages in A4 format. To save the costs, it was printed on very thin paper and with low-coverage ink.

I am looking for a single word or an idiom for very, very thin book pages.

Particular usage:
Despite being printed on ________, this book is still two inches thick!
or
These are some _________ pages. Look how fragile they are, I can almost see through them!

Solution 1:

Also known as 'onion skin' or 'onion paper'. https://en.wikipedia.org/wiki/Onionskin.

Onionskin or onion skin is a thin, light-weight, strong, often translucent paper. Not made from onions, it is named for their thin, papery skins which it superficially resembles. It was usually used with carbon paper for typing duplicates in a typewriter, for permanent records where low bulk was important, or for airmail correspondence. ~ http://cool.conservation-us.org/don/dt/dt2375.html

Onionskin may be an expression used in book publishing in parts of the USA, as pointed out by EL&U member Eric Hauenstein. The term more frequently encountered in the UK is 'India paper', originally suggested here by EL&U member Brian Hooper. India paper is traditionally a thin light paper produced in Asia or in the West in imitation of Asian thin papers. See: https://en.wikipedia.org/wiki/India_paper

This image (below), of the Deluxe (India Paper) single volume edition of Tolkien's Lord of the Rings (http://www.tolkienlibrary.com/tolkien-book-store/NH0013.htm) gives some idea of how translucent India paper is when used in bound books:

This image (below) is of 'Bible Paper' (referenced by EL&U member mikeagg in his answer to this question) in Schuyler Publisher's Quentel Bible. Bible Paper is essentially the same product as India Paper, but has become the standard term for this sort of very thin paper when used in the production of Bibles. Note that at 45gsm, Shuyler claim that this is the most opaque Bible Paper in the market. (https://tresses.wordpress.com/2014/02/17/schuyler-quentel-nasb-review/)

Just for completeness, here (below) is an image showing onionskin paper. Note the distinctive colour. In former times single plain onionskin sheets were interleaved in books to protect images, and also to prevent heavily inked pages containing images adhering to the page opposite. The drying process applied to onionskin paper gives it a very slightly uneven surface that prevents pages sticking together.

Some Ngrams from Google give us some sense of what is going on. Note that simply searching for onionskin paper (or any of its variant spellings) will find matches for typing and writing paper. This question is asking for information about paper used in book publishing, hence the Ngram search should be for something like 'printed on...', as in this example:

Another Ngram makes the point about the dominance of India paper in publishing even more forcefully:

Finally Onionskin (and its variants) up against India paper (case insensitive):

On a final - final - note, there is some confusion being generated in this discussion over what is meant by the term India paper. This explanation from the ABE Book Trading site should help clear matters up:

Solution 2:

Lightweight Offset Paper

Regular folks outside the printing industry normally call this Bible paper or sometimes rice paper, but the technical term used by the printing industry for this lightweight offset paper.

From Wikipedia:

Bible paper is a thin grade of paper used for printing books which have a large number of pages. Technically it is called lightweight offset paper and is a type of woodfree uncoated paper. This paper grade often contains cotton or linen fibres to increase its strength in spite of its thinness.

Solution 3:

This is commonly referred to as Bible paper.

Solution 4:

It might be called India paper:-

a thin tough opaque printing paper [Merriam-Webster]

or Oxford India paper, another term for it.