Halcyon /ˈhalsēən / adj.

Definition: Halcyon is used to describe something characterized by happiness, great success, and prosperity. It is often used to describe an idyllic period in the past that is remembered as better than today. Halcyon is also a noun to identify the White-Breasted Kingfisher, a bird found in Asia.

Etymology: Halcyon first appeared in Greek mythology. The daughter of the winds, Alcyone, threw herself into the sea and transformed into a kingfisher upon learning of the death of her beloved husband. The Greeks referred to such birds as alkyōn or halkyōn. 

According to legend, these birds built their nests at sea, and the wind god provided for calm conditions until their eggs hatched. As a result, the use of halcyon as an adjective to denote calm began in the 15th century and as a noun naming the kingfisher bird in the 14th.

In a Sentence

He recalled his halcyon days at the University.

The halcyon perched on my windowsill and displayed its stunning color.

Last spring, we enjoyed a halcyon escape by the lake.

Synonyms 

At Peace, Booming, Quiet

Antonyms 

Agitated, Stormy