Prop-word

A prop-word is a word with little or no semantic content used where grammar dictates a certain sentence member, e.g., to provide a "support" on which to hang a modifier. The word most commonly considered as a prop-word in English is one (with the plural form ones).

Another term for this concept is "pro-noun" (here "pro" means "for", "instead of"), with a similar concept "pro-verb" "do" (as exemplified by "I will go to the party if you do").