A note not within the 5 lines and 4 spaces of the staff.
Interval knowlege is greatly useful to judge ledger distance and name the note.
For this, it is especially helpful to remember the 1st and top lines of each clef:
Treble 1st Line: E Treble Top Line: F
Bass 1st Line: G Bass Top Line A
High C is 2 ledger lines up from the Treble
Low C is 2 ledger lines down from the Bass