This is the 239th most frequent French word.
sein
The French word "sein" means "breast" or "bosom" in English.
In this sentence, 'sein' is used in the phrase 'au sein de', meaning 'within' or 'at the heart of', referring to being part of or included in the organization.
Elle était au sein de l'organisation pendant de nombreuses années.
She was within the organization for many years.
The word 'sein' figuratively represents the 'heart' or 'interior' of something, in this case, the mountain.
Dans le sein de la montagne se cache une grotte mystérieuse.
Within the heart of the mountain lies a mysterious cave.
Here, 'sein' also appears in the expression 'au sein de', but in this context it emphasizes the core or nurturing environment of the family.