Five orders of ignorance by Phillip G. Armour:

0th Order Ignorance (0OI): Lack of Ignorance
I (provably) know something

1st Order Ignorance (1OI): Lack of Knowledge
I do not know something

2nd Order Ignorance (2OI): Lack of Awareness
I do not know that I do not know something

3rd Order Ignorance (3OI): Lack of Process
I do not know a suitably effective way to find out that I don’t know that I don’t know something

4th Order Ignorance (4OI): Meta-Ignorance
I do not know about the Five Orders of Ignorance

Phillip G. Armour: The five orders of ignorance. Commun. ACM 43 (10): 17-20 (2000)