Not agitated; without losing self-possession:
— He remained serene in the midst of turbulence.
— A serene expression on her face.

synonyms: calm, tranquil, unagitated.

Roget 425: transparent, pellucid, lucid, diaphanous, translucent, tralucent^†, relucent^†; limpid, clear, serene, crystalline, clear as crystal, vitreous, transpicuous^†, glassy, hyaline; hyaloid [Med.], vitreform^†.    ... show more

Roget 826: inexcitable^†, unexcitable; imperturbable; unsusceptible etc. (insensible) 823; unpassionate^†, dispassionate; cold-blooded, irritable; enduring etc. v.; stoical, Platonic, philosophic, staid, stayed; sober, sober minded; grave; sober as a judge, grave as a judge; sedate, demure, cool-headed.    easy-going, peaceful, placid, calm; quiet as a mouse; tranquil, serene; cool as a cucumber, cool as a custard; undemonstrative.    temperate etc. (moderate) 174; composed, collected; unexcited, unstirred, unruffled, undisturbed, unperturbed, unimpassioned; unoffended^†; unresisting.    meek, tolerant; patient, patient as Job; submissive etc. 725; tame; content, resigned, chastened, subdued, lamblike^†; gentle as a lamb; suaviter in modo [Lat.]; mild as mothers milk; soft as peppermint; armed with patience, bearing with, clement, long-suffering.    ... show more

Roget 831: content, contented; satisfied etc. v.; at ease, at one's ease, at home; with the mind at ease, sans souci [Fr.], sine cura [Lat.], easygoing, not particular; conciliatory; unrepining^†, of good comfort; resigned etc. (patient) 826; cheerful etc. 836.    unafflicted, unvexed^†, unmolested, unplagued^†; serene etc. 826; at rest, snug, comfortable; in one's element.    satisfactory, tolerable, good enough, OK, all right, acceptable.    ... show more

Polish: spokojny

Completely clear and fine.