Among them, one of the most cited approach is back translation (briefly, BT) (Sennrich et al., 2016a), which leverages the target-side monolingual data.
a target-to-source translation model (trained on the genuine bitext) is used to translate target-side monolingual sentences into the source domain to generate a set of synthetic bitext, which is then used together with the genuine bitext to train a source-to-target NMT model.