RPKM is a method of quantifying gene expression from RNA sequencing data by normalizing for total read length and the number of sequencing reads.
Mortazavi, A., Williams, B. A., McCue, K., Schaeffer, L., and Wold, B. (2008).
Mapping and quantifying mammalian transcriptomes by rna-seq.
Nat Methods, 5(7):621-628.
Nature Methods link