hi it stand for multitrehhold voltage,this technique is basicaly used for low power cmos circuit design,in which in side a single circuit diffrent transistor used are of deffrent threshold value for optimized power dissipation and speed
MTCMOS - multithreshlod CMOS
normally used 2 Vth,
HVth - high threshold voltage - used for standby mode - avoiding leakage current
LVth - low threshold voltage - used for active mode - for speed and performance