Closed source is an antonym for open source software and refers to any program whose license does not meet the definition of open source software. Generally, it means only the binaries of a computer program are distributed and the license provides no access to the program’s source code, rendering modifications to the software technically impossible for practical purposes.
The source code of such programs is usually regarded as a trade secret of the company.
Access to source code by third parties commonly requires the party to sign a non-disclosure agreement.