A contention ratio is a measure of how widely a network connection is shared. For example, most home DSL/ADSL connections have a contention ratio of 50:1 which means that the 512Kbps is shared between 50 people. Normally, however, only 1 or 2 people are using the connection at the same time so the users still get close to 512Kbps.
Servers with unmetered bandwidth have varying degrees of connection sharing (2:1-10:1). There is always a guaranteed minimum, for example a server with a 5Mbps, 10:1 connection would be guaranteed a minimum of 0.5Mbps.