A sekirei is a superhuman or more like a humanoid with genetic similarities to human’s. A Sekirei should be winged by their Ashikabi so that they can participate in the sekirei-plan with their full extends. Sekirei plan is basically a game where the last one standing wins the game.
Note: Spoilers ahead, the content below contain info that you may not want to read to maintain the suspense of the story, if you are reading manga, as for Anime, the remaining story is not adapted into anime.
Each Sekirei participates in Sekirei plan to win it, which involves defeating other sekirei. The defeat means that they are no longer able to operate any further that is they are terminated. So what is termination?.
In a fight when the dual between two Sekirei happens the defeated sekirei’s crest disappears which actually means Termination of Sekirei, which is same as dead. The rules stated by MBI is that when a sekirei is defeated the Winner Sekirei should guard the corpse until it is recovered by the MBI.
Defeated Sekirei don’t die biologically, but they stop functioning. The recovered body of the sekirei is taken to the MBI HQ where they are kept in special chambers.
In manga the story is almost over, the final round that is 4th round the winner Sekirei attains the power and grants a wish which revives all the Sekirei’s which are terminated, due to which they are restarted.
Also in the final round, Kusano sekirei number 108 has the power to revive the terminated sekirei, thus Reviving some of them.