Are you asking what it's called in ordinary conversation or what you call it in the HTML code?
In HTML4 the dropdown item that's pre-selected, if any, gets an extra attribute selected = "selected" (equivalent to checked = checked when it's a checkbox).
It sounds as if you're in HTML 5 which adds the "placeholder label option" along with "defaultSelected". The difference depends on whether your user is allowed to stay with the default or has to pick something else.
A "placeholder" alone is essentially the same in 4 and 5 except that in 4 it didn't have a special name. It goes in text-input fields-- which may have different names in HTML 5-- to say things like "Smith, John" or "Explain your problem here".