What exactly is meant by the term "library" in the context of RNA/DNA sequencing ?

Question

[K. Bedejova]

Hello ! I am a beginner in the field of bioinformatics and I am currently beginning to work on my master thesis which will focus on gene expression analysis. I'm reading a lot of research papers to give myself an introduction into this field and I've come across the term "library" many times, especially in the context of RNA-seq. Specifically, I learned that one does the following when doing the biochemical part of RNA-seq :

1. RNA extraction
2. Library preparation
3. Sequencing

Could someone please explain to me what exactly is meant by this term "library" ? I tried to search for it on the internet but it's still not clear to me. Many thanks !

[GenoMax]

library is a collection of DNA fragments (from a sample) of a certain size that have special adapters added on their ends so they can be anchored onto a support (beads, glass etc) and then sequenced.

Answer 1

[Pierre Lindenbaum]

It's in fact, a molecular biology term. You're creating a "library" when you're extracting some DNA from a set of cells. One sample can have multiple libraries if the DNA was extracted at different time, with a different protocol, with some different cells, etc...

see wp: https://en.wikipedia.org/wiki/Genomic_library

see also: ILMN "Comprehensive NGS library preparation solutions" https://www.illumina.com/techniques/sequencing/ngs-library-prep.html