Non-proprietary software is, in its widest possible sense, software that has no proprietary restrictions attached to it, particularly the restriction about the access to the source code. Non-proprietary software is the opposite of proprietary software, which is software where the source code has not been made available, or it is protected by restrictive licensing clauses.
Non-proprietary software is an umbrella term that encompasses many different types of software licenses and development schemes, such as Free software, Open Source Software, and even public domain. Another accepted umbrella term is Free/Libre Open Source Software (FLOSS).