Illustrated Glossary of Organic Chemistry

Library (chemical library): A collection of molecules, usually of similar structure, used to test effects of structural variations on (for example) biological activity of drug candidates.

A pharmaceutical company looking for a drug structurally similar to aspirin (but more effective) might synthesize a library of compounds similar to the molecules shown above. This sample library contains eight molecules. The molecule in red is aspirin itself. Note the library members all have some similarity in molecular structure.