This is the 7552nd most frequent Japanese word.
凪
A calm or lull in wind and waves.
Here, '凪' refers to the condition of the sea being calm, with no wind or waves, commonly used to describe weather or sea conditions.
海が静かになり、凪の状態だった。
The sea calmed down, and it was in a state of calmness.
In this sentence, '凪いだ' is used as an adjective describing the air as calm and tranquil.
今日は凪いだ空気の中で散歩を楽しむことができます。
Today, you can enjoy a walk in the calm air.
In this instance, '凪' metaphorically conveys a tranquil or uneventful phase in one's life or situation.