Android is an operating system for mobile devices such as smartphones and tablet computers. It is developed by the Open Handset Alliance led by Google.
With the exception of brief update periods, Android has been available under a free and open source software license since October 21, 2008 until March 2011. Google published the entire source code (including network and telephony stacks) under an Apache License. Google also keeps the reviewed issues list publicly open for anyone to see and comment.
Even though the software is open source, device manufacturers cannot use Google's Android trademark unless Google certifies that the device complies with their Compatibility Definition Document (CDD). Devices must also meet this definition to be eligible to license Google's closed-source applications, including the Android Market.