From metechō (to partake). Denotes sharing, partnership, or participation. Paul uses it to make a sharp contrast between light and darkness.
Paul asks the rhetorical question: 'What fellowship can light have with darkness?' (2 Corinthians 6:14). The expected answer is 'none.' This word draws an absolute line between the kingdom of God and the kingdom of darkness. There can be no partnership, no compromise, no middle ground between truth and falsehood, between Christ and Belial. Believers must choose.