平へい静せい

冷れい静せいN3sense 2
冷静 stresses cool, rational self-control (especially when making decisions), while 平静 covers both outer calm and inner composure more broadly.
穏おだやかN3sense 1
穏やか describes a gentle, mild peacefulness (of weather, personality, or mood), whereas 平静 focuses on the absence of disturbance or agitation.
落おち着つきN1sense 2
落ち着き is the everyday word for a settled, composed manner, while 平静 is a more formal term for the same inner calm, often paired with 保つ or 失う.
静せい寂じゃくN1sense 1
静寂 emphasizes physical silence and stillness in a place, while 平静 refers more to a calm state or atmosphere free of disturbance in general.
平たいらN3
平静 is a formal, often written word for a calm, unshaken state of mind, close in meaning to the figurative sense of 平ら.

Opposites

動どう揺ようN1sense 2
動揺 means being emotionally shaken or unsettled, the direct opposite of the inner composure expressed by 平静.
混こん乱らんN3sense 1
混乱 describes chaos or disorder, contrasting with the tranquil, undisturbed state described by 平静.
パニックN3
平静 means calm composure, the opposite state to the chaotic fear of パニック.
激げき情じょう
平静 describes calm, composed emotional stillness, the opposite of the violent surge of feeling in 激情.

平へい常じょうN1
平常 means 'normal, usual' (as in one's regular state), while 平静 specifically means being calm or composed, not merely 'as usual'.
平へい成せいN1
平成 is the name of a Japanese era (1989-2019) and is pronounced identically to 平静, but the two share no meaning.

Compound of 平 (flat, even, calm) and 静 (quiet, still). The combination naturally conveys a state of calm and stillness.