Synonyms of collected

1 Syllable
cool, staid, sure, still, quiet
2 Syllables
staidest, neutral, tranquil, placid, heaped, poised, cold-fish, easy, staider, sourced
3 Syllables
unanxious, reserved, hang-loose, unflurried, outgiving, received, confident, rational, nonchalant, cool-headed, peaceful, easygoing, serene, unruffled, relaxed, sanguine, neutralist, unfazed, unvexed, variform, iceberg, nerveless, possessed, together, bystanding, acquired, varietal, softspoken, coolheaded, omniform, gathered
4 Syllables
self-controlled, self-restrained, unperturbed, levelheaded, unflappable, impassive, unexcited, aggregate, self-possessed, non-combatant, temperate, introverted, uninvolved, noncombatant, nonaligned, level-headed, middle-of-road, forgathered, deductive
5 Syllables
unagitated, unimpressible, disimpassioned, dispassionate, inexcitable, unperturbable, even-tempered, nonbelligerent
6 Syllables
uncompanionable, nonparticipating