I cannot find a good translation for the computer science term placeholder, which could be defined as a symbol that will later be replaced by some other item (for instance, in the text "Bonjour #, comment allez-vous ?", the # character is a placeholder which will be replaced by the name of the user).
The nearest term I could get to is "un substituant", but that's not a user friendly terminology. I would like to stick to something short and not have to resort to a paraphrase like "le symbole à substituer".