Source : Webster's Revised Unabridged Dictionary (1913)

Membership \Mem"ber*ship\, n.
   1. The state of being a member.

   2. The collective body of members, as of a society.

Source : WordNet®

membership
     n 1: the body of members of an organization or group; "they
          polled their membership"; "they found dissension in
          their own ranks"; "he joined the ranks of the
          unemployed" [syn: {rank}]
     2: the state of being a member