No commercial usage clause
A clause published alongside a copyright notice, usually in lieu of a full licence, with the text simply stating that "you are allowed to use the software in a personal setting but it may only be used commercially with a full license from the author" or words to that effect. This was and is primarily used in cases where a full un-crippled version of software (typically shareware) or full source code is included with a software package that is otherwise meant to be a demonstration copy or a marketing device.
The clause is simply meant to allow people to use the software at will while forcing businesses to either license the executables/source code according to fixed terms drawn out in the document or to contact the author for licensing details. While simple and effective in some ways, the clause leaves "non-commercial" businesses like government agencies and not-for-profit organisations in some limbo as it is usually not made clear if they are considered a commercial entity or not by the author/issuer of licence.
This is especially pertinent since laws, regulations, conventions and perspective of what a commercial entity is differs widely amongst legal jurisdictions and cultural zones.
It may therefore be better for software authors to allow "private use only" or a similarly worded clause to their software and skip the "commercial" part altogether since the recipient may not agree with the author as to what constitutes a commercial organisation or usage, but private is a much less ambiguous term.
In the more recent years it has become more common for people to offer two different licences and price structures on software products according to their intended use, with open source authors in particular barring the use of their products in any commercial situation although the alternative of barring the change of the source code in that situation works as well in most cases.
But even bigger commercial vendors have started doing the same, the "Home and Student" versions of Microsoft Office display prominently a "No commercial usage allowed" on their applications task-bar and some vendors have different usage terms according to its final use, for instance Ability Office cost the same for private or business use, but private users may install the package on five different machine while commercial users are only allowed to install on two. Similarly the current "home" version of eComStation allows the use of the OS on three different PC's concurrently while the "business" version is only allowed on one.