Multicellular organisms have well developed reproductive organs which participate in reproduction. Their whole body does not participate in reproduction unlike unicellular organisms.Although, cell division does occur in reproducive organs in multicelluar organism but other events also occur during reproduction in them. So, cell division can not be a type of reproduction in multicellular organisms. .
Unicellular organism undergo cell division process and increase in number.So, cell division is synonym of reproduction in unicellular organisms.