Epistasis

Epistasis is the interaction between genes at different loci. It was a term invented by William Bateson in 1909.
When Gregor Mendel did his experiments on the sweet pea, he chose seven traits, such as round vs wrinkled peas, and tall vs short plants. He found the plants had pairs of 'factors' (genes) which controlled their appearance (phenotype). However, one pair of factors "never affected the other pairs". But with epistasis, the action of one gene does affect another gene, even one on a different chromosome. 
Use of the term has varied somewhat. Its original meaning was quite narrow: for instance, a gene for "no wings" in "Drosophila" will obviously mask genes for any other aspect of the wings. In general, however, epistasis denotes any departure from independence of the effects of different genetic loci.