of a body of water Free from disturbance by heavy waves:
— A ribbon of sand between the angry sea and the placid bay.

synonyms: quiet, smooth, still, tranquil, unruffled.

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

Not easily irritated:
— Not everyone shared his placid temperament.
— Remained placid despite the repeated delays.

synonyms: equable, even-tempered, good-tempered.