Client access license
A client access license ("CAL") is a commercial software license that allow clients to connect to server software and use their services. Most commercial client software are licensed so that payment is required for each installation. However, some server software, which are accessed by multiple devices or users concurrently, are licensed so that payment is required for each concurrent use. Thus each concurrent use is authorized with a client access license. For example, an instance of Windows Server 2003 for which ten CALs is purchased allows 10 concurrent connections.