noun, feminine/ka.ʁɑ̃.tɛn/plural: quarantaines
a number or group of or about forty; one's forties (period of life between ages 40 and 49); quarantine (isolation to prevent contamination)
“une femme dans la quarantaine” — a woman in her forties
From quarante + -aine, the medical sense a calque of Italian quarantena.