Betrothment

the act or state of being betrothed; engagement.
Historical Examples

The Story of an African Farm (AKA Ralph Iron) Olive Schreiner
Floyd Grandon’s Honor Amanda Minnie Douglas
The Devil’s Elixir E. T. A. Hoffmann
Septimius Felton Nathaniel Hawthorne
English Men of Letters: Coleridge H. D. Traill
Belford’s Magazine, Volume II, No. 8, January, 1889 Various
The Chronicle of the Norman Conquest Master Wace
The Cross of Berny Emile de Girardin

noun
engagement to be married
a mutual promise to marry
n.