An expressed sequence tag (EST) library is a collection of short cDNA (complementary DNA) sequences that are generated from the mRNA (messenger RNA) of an organism. The name "expressed sequence tag" is fitting because these short sequences are "tags" that identify the expressed genes in a particular cell or tissue.
The term "expressed" refers to the fact that the mRNA is transcribed from a gene and has therefore been "expressed" or made into RNA. The term "sequence" refers to the fact that the cDNA sequences are determined by sequencing the mRNA. Finally, the term "tag" refers to the fact that the ESTs are short sequences that can be used to identify expressed genes in a particular cell or tissue.
Therefore, the name "expressed sequence tag library" is fitting for a collection of clones made from mRNA because it accurately describes the nature and function of the library.