sein

/sɛ̃/
1.
nounmasculine

breast (anatomical)

chest (in context of the female breast)

Elle donne le sein à son bébé.

She is breastfeeding her baby.

Synonyms:poitrine, buste
2.
nounmasculine

within, inside (the interior of an organization, group or context; often used in the phrase 'au sein de')

Il y a des tensions au sein du groupe.

There are tensions within the group.

Synonyms:intérieur, milieu