A substantial portion refers to a large or significant part of something. It can be used in various contexts, such as describing a major fraction or percentage of an item, group, population, or concept. For example, if we say that a substantial portion of the world's population lives in Asia, it means that a large number of people from this continent make up a significant part of the global population. Similarly, if someone says they have a substantial portion of their retirement savings invested in stocks, it implies that they have put a considerable amount of money into the market. In summary, "substantial portion" refers to something that is large or important enough to be considered noteworthy within its context.