Oxford English Dictionary Online. You have to subscribe to OED to use the service which is updated quarterly with at least 1000 new and revised entries
Site Hits: 229 today, 217 yesterday, 1,198 this month, 1,378 last month, 303,090 total System Hits: 150,071 today, 162,772 yesterday, 2,888,429 this month, 4,104,428 last month, 1,588,524,526 total