Well its a good question with several nuances in reply. It probably would help to start by knowing what you think it means in the buddhist context.
However i'll give it a shot first off.
First i think its best to realise that closest translation to what the buddha probably meant is ignorance. john Peacock says as much but he thinks many people feel insulted by the word. they shouldn't.
In the buddhist context the basic meaning being referred to by ignorance as in greed aversion and ignorance means ignorance of the teachings of the buddha and ignorance of what the buddha is trying to get people to know. The word delusion is used but even so the meaning intended is exactly the same so i think its best to stick with the right choice of word.
once i heard it said that when someone acts badly it can be put down to ignorance of the dharma. Not just in not knowing about the dharma but in not knowing by experience and firsthand the knowledge that the dharma wants to convey.
for example, when a person is violent or angry towards another its becuase they are ignorant of not self and out interconnectedness. you see? If one were not ignorant; if one truly knew not-self, one would definitely want to restrain any violent or aggressive tendency or maybe not even have any violent or aggressive impulse in the first place.
so that's what's really meant by delusion aka ignorance.
However using the word delusion is also useful. So i use both. I like the word delusion to convey the more mundane aspect of the buddhist path. To not see things correctly but to always be putting our own distorted overlay on the way we see what's going on in the world.
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