This is the 4221st most frequent French word.
déclin
Decline; deterioration or downward trend.
Here, "déclin" refers to a general decrease or deterioration over time, specifically of a civilization.
Le déclin de la civilisation ancienne est étudié par les historiens.
The decline of ancient civilization is studied by historians.
In this case, "déclin" is used to describe a reduction or worsening in a personal ability or state.
Il a noté un déclin dans sa performance physique ces derniers mois.
He noticed a decline in his physical performance in recent months.
Here, "déclin" indicates a swift process of deterioration or worsening in the condition of something physical.