Part is the usual word for "something less than the whole": "Here is part of the treasure." "For my part, I ask nothing." Share also means "part" but specifically refers to that which is allotted or designed for someone; it emphasizes the receiver: "I want my fair share of the money." "We divided the food into equal shares." Portion means a "part that is given for a purpose": "My portion of the job is to entertain the visiting players." Related words include piece, segment, section, sector, division, fragment, and component.
Dictionary of problem words and expressions. Harry Shaw. 1975.