The Collaborative International Dictionary of English v.0.48:

Stand-by \Stand"-by`\ (st[a^]nd"b[imac]`), n.
   One who, or that which, stands by one in need; something upon
   which one relies for constant use or in an emergency.
   [1913 Webster]

WordNet (r) 3.0 (2006):

standby
    adj 1: ready for emergency use; "a standby generator"; "a
           standby crew"
    n 1: something that can be relied on when needed
    2: an actor able to replace a regular performer when required
       [syn: understudy, standby]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "standby":
   Maecenas, abettor, admirer, advocate, aficionado, angel, apologist,
   backer, bit player, buff, champion, defender, dependence,
   encourager, endorser, exponent, extra, fan, favorer, figurant,
   figurante, friend at court, lover, mainstay, maintainer, mute,
   paranymph, partisan, patron, promoter, protagonist, reliance,
   second, seconder, sectary, sider, spear-carrier, sponsor, stalwart,
   stand-in, substitute, supe, super, supernumerary, support,
   supporter, supporting actor, supporting cast, sustainer,
   sympathizer, understudy, upholder, votary, walk-on,
   walking gentleman, well-wisher