CALM, TRANQUIL, SERENE, PLACID, PEACEFUL mean quiet and free from disturbance.

CALM often implies a contrast with a foregoing or nearby state of agitation or violence.

e.g.the protests ended, and the streets were calm again

TRANQUIL suggests a very deep quietude or composure.

e.g.the tranquil beauty of a formal garden

SERENE stresses an unclouded and lofty tranquility.

e.g.watched the sunset of a serene summer's evening

PLACID suggests an undisturbed appearance and often implies a degree of complacency.

e.g.remained placid despite the criticism

PEACEFUL implies a state of repose in contrast with or following strife or turmoil.

e.g.grown peaceful in old age

source: Swimming Anime Love How Hes Calm GIF - Find & Share on GIPHY

1. (of a person, action, or manner) not showing or feeling nervousness, anger, or other emotions:

〈人･心･態度などが〉落ち着いた, 冷静な

e.g. his voice was calm

e.g. keep calm, she told herself.

(of a place) peaceful, especially in contrast to recent violent activity:

〈社会情勢などが〉平穏な

e.g. the city was reported to be calm, but army patrols remained.

2. (of the weather) pleasantly free from wind:

〈天候などが〉風のない, 平穏な(↔ stormy)

e.g. the night was clear and calm.

(of the sea) not disturbed by large waves.

〈海･湖などが〉穏やかな, 波の立たない(↔ rough)

1. the absence of violent or confrontational activity within a place or group:

平穏, 平静, 平和 (!具体例ではa ～; その際修飾語を伴う)

e.g. the elections proceeded in an atmosphere of relative calm

e.g. in singular : an edgy calm reigned in the capital.

the absence of nervousness, agitation, or excitement in a person:

(心･態度などの)冷静, 落ち着き, 平静

e.g. his usual calm deserted him.

2. the absence of wind:

無風状態, なぎ

e.g. in the center of the storm calm prevailed.

still air represented by force 0 on the Beaufort scale (less than 1 knot).

〘気象〙 静穏〘風が秒速0.2m以下〙

(often calms) an area of the sea without wind.

verb with object

make (someone) tranquil and quiet; soothe:

…を静める, 穏やかにする, なだめる(down)

e.g. I took him inside and tried to calm him down

e.g. he lit a cigarette to calm his nerves

e.g. (as adjective calming) : a cup of tea will have a calming effect.

no object (calm down) (of a person) become tranquil and quiet:

静まる, 落ち着く, 治まる(down)

e.g. gradually I calmed down and lost my anxiety.

see storm.

late Middle English: via one of the Romance languages from Greek kauma ‘heat (of the day)’.