“非法同居”常见的英文表达有 "illegal cohabitation" 或 "unlawful cohabitation" 。
“illegal”和“unlawful”都有“非法的、不合法的”意思；“cohabitation”指“同居；共同生活” 。例如：They were charged with illegal cohabitation.（他们被指控非法同居 。）