SILENT, TACITURN, RETICENT, RESERVED, SECRETIVE mean showing restraint in speaking.

SILENT implies a habit of saying no more than is needed.

e.g. the strong, silent type

TACITURN implies a temperamental disinclination to speech and usually connotes unsociability.

e.g. taciturn villagers

RETICENT implies a reluctance to speak out or at length, especially about one's own affairs.

e.g. was reticent about his plans

RESERVED implies reticence and suggests the restraining influence of caution or formality in checking easy informal conversational exchange.

e.g. greetings were brief, formal, and reserved

SECRETIVE, too, implies reticence but usually carries a suggestion of deviousness and lack of frankness or of an often ostentatious will to conceal.

e.g. the secretive research and development division

(of a person) reserved or uncommunicative in speech; saying little:

｟かたく｠ 無口な, 口数の少ない; むっつりとした(↔ talkative)

e.g. after such gatherings she would be taciturn and morose.

taciturnity |ˌtasəˈtərnədē| noun

taciturnly |ˈtasəˌtərnlē| adverb

late 18th century: from Latin taciturnus, from tacitus (see tacit).