Delusion is the 10th and final fetter. It's also called ignorance. There could be a difference between the two, I'm not sure. Delusion is not understanding the nature of one's own suffering, or not understanding the 4 N.T.
There is a subtle distinction between delusion (
moha) and ignorance (
avijja) in the Pali Canon:
"Ignorance (
avijja)
Within the Theravada tradition,
moha is classified as one of the three unwholesome roots, which are the root or source of all of the other unwholesome mental factors.
In this tradition,
moha is considered to be synonymous with
avijja, but the terms are used in different contexts. Bhikkhu Bodhi explains:
Ignorance (
avijja) is actually identical in nature with the unwholesome root "delusion" (
moha). When the Buddha speaks in a psychological context about mental factors, he generally uses the word "delusion" (
moha); when he speaks about the causal basis of samsara, he uses the word "ignorance" (
avijja). Thus, the term
avijja is used when identifying the first causal link in the twelve links of dependent origination, and
moha is used when discussing the mental factors."
Source: Wikipedia
Hopefully Bodhi is correct on this point.