Deon is the neuter participle of dei (it is necessary/must), used substantively to mean 'the needful thing' or 'what is fitting.' It appears in Acts 17:29 and 1 Timothy 5:13 in the sense of what is proper. The word expresses necessity arising from the nature of things — what is morally fitting, practically required, or divinely ordained.
Deon grounds Christian ethics in necessity rather than mere preference. There are things that must be done — not because of external compulsion but because of the nature of God and reality. Paul's ministry was driven by deon: 'Woe to me if I do not preach the gospel!' (1 Corinthians 9:16). The church needs a recovery of duty-language: some things are not options. Love for neighbor is a deon. Justice for the oppressed is a deon. The worship of God is a deon.