Seed is not rank. Seed is a prediction of a player's position in that tournament.
Most of the time it IS the same according to the rank, but in Wimbledon with its very specialized surface, it is calculated by a different (objective) formula, so say a #10 ranked player who's done great on grass can be seeded higher than his rank, say #6 and vice versa.
The origin of the word may come from "scattering" (like what you do with seeding plants) so that the best players don't face each other early.
From the journal American Lawn Tennis of 1898: "Several years ago, it was decided to 'seed' the best players through the championship draw in handicap tournaments so that the players in each class shall be separated as far as possible one from another."
Source(s): Seed and love (tennis)