Nouns ending in -té are usually feminine.
The suffix marks an abstract quality.
Is unanimité masculine or feminine?
l'unanimité
The rule behind it
91.3% of -té nouns are feminine
In context
Sa proposition idiote fut adoptée à l'unanimité.
His foolish proposal was approved unanimously.
Cette proposition a été approuvée à l'unanimité.
This proposal was unanimously approved.
La motion fut adoptée à l'unanimité.
The motion was carried unanimously.