Strain or line: refers to a pool or colony of individuals or cultured cells of a desired genotype or phenotype that is mostly homogeneous and can be bred and/or produced in perpetuity for research or commercial purposes. "Strain" tends to be used more for microorganisms and "line" tends to be used for more complex multicellular organisms (or cultured cells derived from multicellular organisms), but the two words can be used more or less interchangeably.