Cooperative Proof of Work :) Because it is still randomised by PoW, but frequency of block production is enforced cooperatively, both by the witnesses themselves, who should use the same exact algorithm to decide which block to approve, and by the users, who decide which witness operator is the most worthy of the pay.

By the way, in Calibrae, users will be incentivised after signup, to vote on (one, only one is allowed, to stop collusion) a witness, by having the same amount extra added to their reputation value as the initial account creation requires (and this initial stake will be small and the accounts entirely unsuitable for use by bot operators, something like 48 votes per day and unrealisable minimum stake balance - power down is never allowed - and the reputation reduces the vote power, the only way to gain it is by being upvoted or followed).