The Collaborative International Dictionary of English v.0.48:

Plasmid \Plas"mid\, n. (Molecular genetics)
   A piece of DNA, usually circular, functioning as part of the
   genetic material of a cell, not integrated with the
   chromosome and replicating independently of the chromosome,
   but transferred, like the chromosome, to subsequent
   generations of daughter cells. In bacteria, plasmids often
   carry the genes for antibiotic resistance; they are exploited
   in genetic engineering as the vehicles for introduction of
   extraneous DNA into cells, to alter the genetic makeup of the
   cell. The cells thus altered may produce desirable proteins
   which are extracted and used; in the case of genetically
   altered plant cells, the altered cells may grow into complete
   plants with changed properties, as for example, increased
   resistance to disease.
   [PJC]

WordNet (r) 3.0 (2006):

plasmid
    n 1: a small cellular inclusion consisting of a ring of DNA that
         is not in a chromosome but is capable of autonomous
         replication [syn: plasmid, plasmid DNA]