сдержанный

1) General subject: brittle, button down, button-down, buttoned down, buttoned up, chary, chaste, checked, close, close lipped, close mouthed, close tongued, close-lipped, close-mouthed, close-tongued, collected, composed, contained, continent, continent (в словах, поступках), decent, demure, discreet, distant, dry, forbearing, frozen, guarded, inaccessible, incommunicative, inhibited, insular, low key, low profile, low-key, low-profile (о позиции, поведении), measured (о речи), mim, moderate, modest, prudent, qualified, quiet, repressed, reserved, restrained, reticent, retiring, sedate, self composed, self contained, self restrained, self-composed, self-contained, self-restrained, sober minded, sober-minded, sobersided (слишком), stand off, stand offish, stand-off, stand-offish, strange, suppressed, tempered, tight (в обращении), unaffectionate, unbroken (об обещании и т. п.), undemonstrative, unenthusiastic (о приеме), unexpansive, unsociable, muted (lacking enthusiasm), understated (о вещах, напр. an understated suit - подчеркнуто скромный, но элегантный костюм), minimalistic, bottled up, curt, cold-minded

2) American: preppy

3) Diplomatic term: soft

4) Psychology: self-collected

5) Business: dominated

6) Aviation medicine: kolytic, sober

7) Makarov: frosted, incommunicable, indrawn, lean (о стиле, слоге), temperate, unbending, undemonstrative (о человеке, чувствах и т.п.)

8) Idiomatic expression: as cool as cucumber