place·hold·er (plās′hōl′dər)
n.
-
One who holds an office or place, especially as a deputy, proxy, or appointed government official.
-
In a mathematical or logical expression, a symbol that may be replaced by the name of any element of a set.
-
In the decimal form of a number, a digit that is not significant.
https://www.thefreedictionary.com/placeholder