AbstractIn this paper we study some classes of infinite words generalizing episturmian words, and analyse the relations occurring among such classes. In each case, the reversal operator R is replaced by an arbitrary involutory antimorphism ϑ of the free monoid A∗. In particular, we define the class of ϑ-words with seed, whose “standard” elements (ϑ-standard words with seed) are constructed by an iterative ϑ-palindrome closure process, starting from a finite word u0 called the seed. When the seed is empty, one obtains ϑ-words; episturmian words are exactly the R-words. One of the main theorems of the paper characterizes ϑ-words with seed as infinite words closed under ϑ and having at most one left special factor of each length n≥N (where N is some nonnegative integer depending on the word). When N=0 we call such words ϑ-episturmian. Further results on the structure of ϑ-episturmian words are proved. In particular, some relationships between ϑ-words (with or without seed) and ϑ-episturmian words are shown.