Definition of "crowning"

It seems that now [the Devil] was driving Alison hard. She had been remiss of late — fewer souls sent to hell, less zeal in quenching the Spirit, and, above all, the crowning offense that her bairn had communicated in Christ's kirk.

1902, John Buchan, The Outgoing of the Tide

Friday night’s crowning victory at The Hawthorns was the 25th in 30 league matches since Antonio Conte’s decisive re-gearing of his team in September, the tactical switches that have coaxed such a thrilling run from this team of bolt-ons and upcycled squad players, most notably Victor Moses, who was dredged out of the laundry bin in the autumn to become a key part of the title surge.

2017 May 13, Barney Ronay, “Antonio Conte’s brilliance has turned Chelsea’s pop-up team into champions”, in the Guardian‎

Lilian Greenwood has ranked boosting diversity and inclusivity among her crowning achievements from her two-year stint chairing the House of Commons Transport Select Committee.

2020 May 20, “Network News: A legacy of greater diversity in transport”, in Rail, page 13

There were outflankings and crownings of hills by numbers of thirteen and seventeen men, that made one hold one's breath.

1860, Fraser's Magazine, volume 61, page 711

When your baby’s head remains visible without slipping back in, it is known as crowning.

2012 April, Baby Crowning‎, American Pregnancy Association