The Collaborative International Dictionary of English v.0.48:

Indefatigability \In`de*fat`i*ga*bil"i*ty\
   ([i^]n`d[-e]*f[a^]t`[i^]*g[.a]*b[i^]l"[i^]*t[y^]), n.
   The state of being indefatigable.
   [1913 Webster]

WordNet (r) 3.0 (2006):

indefatigability
    n 1: tireless determination [syn: indefatigability,
         indefatigableness, tirelessness]