MegaROM means generally a cartridge with a ROM of at least 1 mega-bit (128kB or more). These use a mapper intended to extend the memory to more than 64kB. However, there may be cartridges using such mapper while their ROM size is 64kB or less.
MegaRAM is the term to designate a cartridge that contains RAM mapped as a MegaROM. Data is lost when MSX is off.
It allows to execute ROM images exactly as the original cartridge with same ROM mapper, and according to the MegaRAM type and software used, it allows to also execute them exactly as the original cartridge in 80% of cases when ROM mapper is different (using patches).