restrained

adjective

1. Kept within sensible limits: conservative, discreet, moderate, reasonable, temperate. See plain/fancy, restraint/unrestraint.
2. Not showy or obtrusive: inobtrusive, quiet, subdued, tasteful, unobtrusive. See plain/fancy.
3. Tending to keep one's thoughts and emotions to oneself: controlled, inhibited, noncommittal, reserved, self-controlled, self-restrained. See restraint/unrestraint.

Definition: calm, quiet
Antonyms: assured, bold, confident, emotional, extroverted, outgoing

The adjective has 6 meanings:

Meaning #1: cool and formal in manner
  Synonyms: reserved, reticent, unemotional

Meaning #2: under restraint
  Antonym: unrestrained (meaning #1)

Meaning #3: marked by avoidance of extravagance or extremes
  Synonym: moderate

Meaning #4: not showy or obtrusive
  Synonym: quiet

Meaning #5: free from ostentation or pretension
  Synonyms: modest, unostentatious

Meaning #6: prudent
  Synonym: guarded