Freemium is a business model where a free product or service is provided along with optional premium upgrades.

It was popularised in the 1980s through software products, where a limited version was given away for free on disc as an incentive to upgrade to a more extensive, paid option.

Example
Today, the model is frequently used for paywalled websites, on which a limited amount of information is available before readers are required to subscribe. It is also used in gaming, whereby in-game currency for premium items can be bought with real-world money. [1]