The Collaborative International Dictionary of English v.0.48:

247 \24/7\ adj. & adv. (tw[e^]n"t[=e]f[^o]r-s[e^]v"en) [From 24
   hours per day, 7 days per week.]
   Without interruption; non-stop; continuous; as, the computer
   manufacturer provides 24/7 technical help.

   Syn: twenty-four-seven, 7/24
        [PJC]

WordNet (r) 3.0 (2006):

24/7
    n 1: uptime is 24 hours a day 7 days a week