Delusionism is a philosophy held by Wikipedians who pursue a middle ground between the attitudes of deletionism and inclusionism (and possibly exclusionism). Delusionists hold that labels of "deletionist" and "inclusionist" hinder the development of balanced policies regarding what should be in the Wikipedia, and often favour the use of precedents to establish trustworthy grounds for vetting articles.
In practice, it would seem that most people do indeed tend to fall into the deletionist and inclusionist groupings. Some say that a central view to deletion policy would lead to a more relaxed editing atmosphere, but others yet question whether that is a valid concern for Wikipedia.
Some call delusionists delusional (hence the name) for seeking a neutral middle ground between two very different extremes, resulting in some users being offended at the term Delusionist entirely, feeling that it casts a negative quality on their beliefs, and prefer the terms Inletionist and Inletionism. Others view the root of the name as referring to the extreme "delusional" views of editors who are deletionists and inclusionists.