Answer: mmd

AnswerCrossword Clue
MMDYear around the middle of our millennium
MMDL squared
MMDC x XXV
MMDCCL x X
MMDEnd of the 25th century
MMD2500, in old Rome
MMDRoman 2,500
MMDSci-fi year
MMDRoman square?
MMDMid-millennium year
MMDTwice MCCL