The principal groomsman at a wedding.

Roget 890: friend, friend of one's bosom; alter ego; best friend, bosom friend, soulmate, fast friend; amicus [Lat.]; usque ad aras [Lat.]; fidus Achates [Lat.]; persona grata.    acquaintance, neighbor, next-door neighbor, casual acquaintance, nodding acquaintance; wellwisher.    favorer, fautor^†, patron, Mecaenas; tutelary saint, good genius, advocate, partisan, sympathizer; ally; friend in need etc. (auxiliary) 711.    comrade, mate, companion, familiar, confrere, comrade, camarade^†, confidante, intimate; old crony, crony; chum; pal; buddy, bosom buddy; playfellow, playmate, childhood friend; bedfellow, bedmate; chamber fellow.    associate, colleague, compeer.    schoolmate, schoolfellow^†; classfellow^†, classman^†, classmate; roommate; fellow-man, stable companion.    best man, maid of honor, matron of honor.    compatriot; fellow countryman, countryman.    shopmate, fellow-worker, shipmate, messmate^†; fellow companion, boon companion, pot companion; copartner, partner, senior partner, junior partner.    Arcades ambo Pylades and Orestes Castor and Arcades ambo Pylades and Orestes Castor and Pollux^†, Nisus and Euryalus [Lat.], Damon and Pythias, par nobile fratrum [Lat.].    host, Amphitryon^†, Boniface; guest, visitor, prot_eg_e.    ... show more

Roget 903: marriage, matrimony, wedlock, union, intermarriage, miscegenation, the bonds of marriage, vinculum matrimonii [Lat.], nuptial tie.    married state, coverture, bed, cohabitation.    match; betrothment etc. (promise) 768; wedding, nuptials, Hymen, bridal; espousals, spousals; leading to the altar etc. v.; nuptial benediction, epithalamium^†; sealing.    torch of Hymen, temple of Hymen; hymeneal altar; honeymoon.    bridesmaid, bridesman^†, best man; bride, bridegroom.    married man, married woman, married couple; neogamist^†, Benedict, partner, spouse, mate, yokemate^†; husband, man, consort, baron; old man, good man; wife of one's bosom; helpmate, rib, better half, gray mare, old woman, old lady, good wife, goodwife.    feme [Fr.], feme coverte [Fr.]; squaw, lady; matron, matronage, matronhood^†; man and wife; wedded pair, Darby and Joan; spiritual wife.    monogamy, bigamy, digamy^†, deuterogamy^†, trigamy^†, polygamy; mormonism; levirate^†; spiritual wifery^†, spiritual wifeism^†; polyandrism^†; Turk, bluebeard^†.    unlawful marriage, left-handed marriage, morganatic marriage, ill-assorted marriage; mesalliance; mariage de convenance [Fr.].    marriage broker; matrimonial agency, matrimonial agent, matrimonial bureau, matchmaker; schatchen [G.].    ... show more

Dutch: getuige voor de bruidegom
Polish: świadek