A check (also check mark, tick) is a dipper-shaped mark.
The same is true of the verb; although to check mark is not used in this sense, simply to mark or to check is. Sometimes, off is added to the verb, thus check off, tick off or mark off.
Sometimes, especially on printed forms, printed documents, and computers, these check marks and tick marks are in squares.
If the user clicks a check box that has a checkmark in it, the checkmark may be removed.
In this case the checkmark would appear as if it were disabled.
If a check box is configured to assume one of three states when it's clicked, that is, if the ThreeState property of a check button is set to True, when the user clicks such a button, the button acquires one of the available three states, which are Checked, Unchecked, or Indeterminate.