任何可用于系统筛选和分离特定分子的大的组织化的生物分子的聚合。文库中生物分子的类型包括遗传物质如：基因、cDNA、小片段蛋白质肽和小分子化合物。例如，一个cDNA文库包括单个组织中所有的cDNA,可以筛选出与已知DNA片段杂交的分子。这被称为cDNA文库。一个包含可表达为蛋白质的基因的文库称为cDNA表达文库，它可以用来筛选结合特定抗体的蛋白。一个由一系列小分子DNA、RNA、多肽或其它试剂以多种方式组合而成的大量变体的文库称为"组合文库"。同样，一个以计算机为基础的药物结构集合可以用来设计选择那些小分子，它们的大小正适合插入已知蛋白。这样的文库称为"虚拟文库"。

Any large, organized collection of biomolecules that can be systematically screened for particular properties, and from which any positive match can be isolated. The types of biomolecules found in libraries include genetic material such as genes or cDNA, proteins in the form of small fragments called peptides, and small molecule compounds. A cDNA library could contain all cDNAs from a single tissue, for example, and could be screened for the ability to hybridize to a fragment of known DNA. This would be referred to as a cDNA library. A library containing the genes that could be expressed as proteins would be referred to as a cDNA expression library, which could be screened for a protein that binds a particular antibody. A library synthesized from a small set of fragments of DNA, RNA, peptides or other reagents that are combined in a variety of ways to generate a large number of variants is known as a “combinatorial library.” Similarly, a computer-based collection of drug structures could be designed to select those molecules of a size that would fit a particular pocket in a known protein. This library would be an example of a “virtual library.”