Market Saturation is the point at which there are few consumers that are still interested in buying a product because those who were ever likely to already have done so. Saturation can be said to exist for all similar products in a market.
This may call for different strategies which could keep a company going. One is that products can be made to wear out after a certain amount of time and need replacement. Another is that the business can shift its focus to subscription or service-based income.
Market penetration is a related topic, which is either the amount of consumers in the target market that have bought a product, or the proportional amount of products sold relative to the total of similar products sold. Products can also be modified or updated to appeal to more people or to resell to the same demographic that purchased the product the first time.