Delirium is a sudden change in how well a person can pay attention and stay aware of their surroundings, along with a change in thinking. It develops over hours or days and tends to vary in severity during the day.
Scotland's SIGN guideline gives this definition.
The 4AT, a short bedside test for delirium, checks attention by asking the person to say the months of the year backwards, starting from December. Saying 7 or more months correctly scores 0 on this part, meaning no sign of a problem.
Other memory and thinking problems can also make this task hard, not only delirium. That is why the 4AT also asks about sudden change.