Relex is a term of criticism saying that someone has naïvely or unreflectively imitated their native language too closely in creating a conlang. Many conlangers' first attempts at conlanging are relexes of their native language on one level or another. Also occasionally called "codes" or "cipherlangs".
Though strictly speaking the term refers to replacing the vocabulary of a natlang and nothing else, in practice it can refer to any extensive borrowing. Usually it refers to borrowing from one's native language, and usually it is a derogative term.