Term used in the CGEL for the clause that can be equated to a dummy subject 'it', e.g.
It is a mistake to eat eclairs in bed.
In most cases it might have been the subject instead:
To eat eclairs in bed is a mistake.
It has been extraposed from subject position to the end of the clause, after other complements. This distinguishes it from the displaced subject of a dummy 'there' clause, which is merely displaced past the verb: