The irmos is the initial verse of each individual ode in a canon, sung by the choir; from the Greek verb "to tie" or "to link." The irmos presents a pattern for all of the troparia which follow within a given ode. It also gives its name to the irmologic forms of Byzantine chant.
"The irmos links (thus the Greek name) the theme of the canon with the theme of the Biblical Ode which corresponds to the ode of the canon—because originally, the troparia of a canon were interspersed between the final verses of the Biblical Odes."
- F.L. Cross & E.A. Livingstone, eds., The Oxford Dictionary of the Christian Church, 2nd Ed. (Oxford: Oxford University Press, 1989), s.v. "Heirmos," p 630.