Is it ensure, insure, or assure?
before vowels mult-, word-forming element meaning "many, many times, much," from comb. form of Latin multus "much, many," from PIE *ml-to-, from root *mel- "strong, great, numerous" (cf. Latin melior "better," Greek mala "very, very much"). Many words that use it (multinational, etc.) are 20c. coinages.
Many; much; multiple: multiarticular.
More than one: multiparous.
More than two: multipolar.
A prefix that means "many" or "much," as in multicellular, having many cells.