"Delusion refers to a belief in something that contradicts reality. In Buddhism， delusion is a lack of awareness of the true nature or Buddha-nature of things， or of the true meaning of existence. According to the Buddhist outlook， we are deluded by our senses -- among which intellect （discriminating， discursive thought） is included as a sixth sense. Consciousness， attached to a sense， leads us into error by causing us to take the world of appearances for the world of reality， whereas in fact it is only a limited and fleeting aspect of reality." （Shambhala Dictionary of Buddhism and Zen.）
"（Delusion is） the fundamental error which brings about existence. The word is used in the common sense of the most basic cause in the development of discriminations. It is the initial perception of either existence or non-existence when in reality there is neither one nor the other. This topic is treated at very great length in the Surangama Sutra."
Buddhism of Wisdom & Faith/ excerpts:
"Delusion （ignorance） is the source of all afflictions. Greed and anger stem from delusion， as do pride， doubts and 'wrong views'. For example， when we have such thoughts as: 'I have few equals in hard and assiduous cultivation， even among the ranks of monks and nuns' -- this is the affliction of pride and self-love. If， on the other hand， we develop such thoughts as: 'The Land of Ultimate Bliss is so well-adorned， while I have heavy karma and few merits; how do I know I will achieve rebirth there?' -- this is the affliction of doubt. Wrong views consist of seeing and understanding in a wicked and grasping manner. They include five types: Wrong Views of the Body， One-Sided Views， Wrong Views Not Consistent with the Dharma， Wrong Views Caused by Attachment to one's own Erroneous Understanding and Wrong Views （understanding） of the Precepts. For example， during cultivation a thought suddenly arises: 'My health has always been poor， and today I feel tired， worn out; if I continue to recite the Buddha's name， I may fall ill.' This is an example of a Wrong View of the Body. Or else， we think: 'Death is like a candle which has gone out; if there was a previous life， how come I cannot remember it? It is better for me to follow Taoism， to prolong my life and avoid death.' This is an instance of a One-Sided View. Or else， we wonder why someone who had performed only good deeds had a short life and met a violent death， while others who had committed numerous transgressions lived long lives and died peacefully; thus， cultivation brings no benefit whatsoever! These are instances of Wrong Views Not Consistent with the Dharma， and failure to understand that the law of cause and effect spans many lifetimes. Or else， someone might think， 'I used to follow the externalist practice of circulating energy currents， and was achieving results in barely a few months. How is it that I have been reciting the Buddha's name for a long time， but have not yet seen any change?' This is an instance of Attachment to one's own Erroneous Understandings. Or else， we may think: 'In other religions， people who hurt and kill other sentient beings can still be reborn in paradise; therefore， in seeking rebirth in the Pure Land， there is no need to keep the precept against killing.' This is an instance of Wrong View of the Precepts. The karma of delusion takes numerous forms. The Pure Land practitioner should， first and foremost， follow the sutras and put his entire faith in them. If he fails to understand certain passages， he should seek out good spiritual advisors for an explanation. Delusion can easily lead the practitioner astray when he is challenged by other teachings and ideologies."
NOTES:
According to Mahayana teaching， there are， in total， 52 levels of ignorance （delusion）， in ascending order of subtlety.
"The real problem for Buddhist practitioners is... spiritual ignorance， which is not just an intellectual defect but a lack of understanding of reality which can be overcome by meditational insight."