Designated for assignment

From BR Bullpen

A player is designated for assignment when his team wants to replace him on the active roster and he is out of options. The team thus announces its wish to send the player to the minor leagues and places his name on waivers. If the player is claimed, a trade may be worked out between the two teams. If the player is unclaimed, he can either accept the minor league assignment, or opt to become a free agent immediately.

Teams have the right to assign a player whose options have all been used outright to the minors only once during his career. This can only be done if his major league service time does not exceed a certain number of days. If he cannot be outrighted, the "designated for assignment" procedure kicks in. Of course, if a team thinks it has no further use for this player even in its minor league system, it can simply hand him his unconditional release. This results in the player immediately becoming a free agent.