placeholder
Something used or included temporarily or as a substitute for something that is not known or must remain generic; that which holds, denotes or reserves a place for something to come later.
Example
“This data is a placeholder, so you'll want to include the real numbers as soon as you have them.”
Other meanings
A non-editable caption initially displayed in a blank text box to indicate its function.
Where this comes from
This definition is adapted from “placeholder” on Wiktionary, by its contributors, published under CC BY-SA 4.0. This page is published under the same licence.