In X-ray crystallography, this parameter is used to express the extent of agreement between theoretical calculations and the measured data; the lower the R-factor, the better the fit (R means either Residual or Reliability).

A transmissible plasmid that carries genes coding for resistance to several different antibiotics. Also called R-plasmid.

R-factor is an old name for a plasmid that codes for antibiotic resistance. Often, R-factors code for more than one antibiotic resistance factor: genes that encode resistance to unrelated antibiotics may be carried on a single R-factor. Many R-factors can pass from one bacterium to another through bacterial conjugation and are a common means by which antibiotic resistance spreads between bacterial species, genera and even families.

In crystallography, the R-factor (sometimes called residual factor or reliability factor) is a measure of the agreement between the crystallographic model and the experimental X-ray diffraction data.