NOUN (3)

1. the period of time during which a regent governs;

2. the period from 1811-1820 when the Prince of Wales was regent during George III's periods of insanity;

3. the office of a regent;

The Collaborative International Dictionary of English v.0.48:

Regency \Re"gen*cy\ (r?*jen*s?), n.; pl. Regencies (-s?z).
   [CF. F. r['e]gence, LL. regentia. See Regent, a.]
   1. The office of ruler; rule; authority; government.
      [1913 Webster]

   2. Especially, the office, jurisdiction, or dominion of a
      regent or vicarious ruler, or of a body of regents;
      deputed or vicarious government. --Sir W. Temple.
      [1913 Webster]

   3. A body of men intrusted with vicarious government; as, a
      regency constituted during a king's minority, absence from
      the kingdom, or other disability.
      [1913 Webster]

            A council or regency consisting of twelve persons.
                                                  --Lowth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

regency
    n 1: the period of time during which a regent governs
    2: the period from 1811-1820 when the Prince of Wales was regent
       during George III's periods of insanity
    3: the office of a regent

Moby Thesaurus II by Grady Ward, 1.0:

182 Moby Thesaurus words for "regency":
   absolute monarchy, aedileship, agency, agentship, archbishopric,
   archiepiscopacy, archiepiscopate, aristocracy, assignment,
   autarchy, authority, authorization, autocracy, autonomy, bishopric,
   brevet, care, chairmanship, chancellery, chancellorate,
   chancellorship, charge, chiefery, chiefry, chieftaincy,
   chieftainry, chieftainship, coalition government, colonialism,
   commission, commissioning, commitment, commonwealth, consignment,
   constitutional government, constitutional monarchy, consulate,
   consulship, cure, deanery, delegated authority, delegation,
   democracy, deputation, devolution, devolvement, dictatorship,
   dictature, directorship, dominion rule, duarchy, duumvirate,
   dyarchy, embassy, emirate, empowerment, entrusting, entrustment,
   episcopacy, errand, executorship, exequatur, factorship,
   federal government, federation, feudal system, full power,
   garrison state, gerontocracy, governorship, headship, hegemony,
   heteronomy, hierarchy, hierocracy, home rule, jurisdiction,
   leadership, legation, license, lieutenancy, limited monarchy,
   lordship, magistracy, magistrateship, magistrature, mandate,
   martial law, masterdom, mastership, mastery, mayoralty, mayorship,
   meritocracy, metropolitanate, metropolitanship, militarism,
   military government, mission, mob rule, mobocracy, monarchy,
   neocolonialism, nobility, ochlocracy, office, oligarchy,
   pantisocracy, papacy, pashadom, pashalic, patriarchate, patriarchy,
   plenipotentiary power, police state, pontificality, pontificate,
   popedom, popehood, popeship, power of attorney, power to act,
   prefectship, prefecture, premiership, presidency, presidentship,
   prime-ministership, prime-ministry, princedom, princeship,
   principality, proconsulate, proconsulship, procuration,
   protectorate, protectorship, provostry, provostship, proxy,
   pure democracy, purview, rectorate, rectorship, regentship,
   representative democracy, representative government, republic,
   responsibility, ruling class, seigniory, self-determination,
   self-government, seneschalship, seneschalsy, sheikhdom,
   sheriffalty, sheriffcy, sheriffdom, shrievalty, social democracy,
   stratocracy, supervisorship, suzerainship, suzerainty, task,
   technocracy, thearchy, theocracy, totalitarian government,
   totalitarian regime, triarchy, tribunate, triumvirate, trust,
   trusteeship, tyranny, vicarious authority, vizierate, viziership,
   warrant, welfare state