Artificially constructed cloning vector containing the cos gene of the lambda phage. Cosmids can be packaged in lambda phage particles for infection into E. coli; this permits cloning of larger DNA fragments (up to 45 kb) that can be introduced into bacterial hosts in plasmid vectors.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.