A Freemium is a  pricing model applied to a software or online applications which provide a limited but permanent use of the product indefinitely, (as opposed to a trial version which offers full access temporarily and then cuts off access at the end of this period).

 Customers and clients  can then pay to ‘upgrade’ to the full version (or versions at several different price points with incrementally added features). This is a great model for organizations to use it as sales strategy in order to gain new clients, which will boost the organization’s revenue.  

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Use in a sentence:
A popular system is freemium.