I understand a Program as to be a set of (technically or otherwise) related Projects with a particular aim to realize strategic, long-term benefits (e.g. functionalities in operation, new technologies, profits, research objectives, etc.) for stakeholders thanks to the realization of a Product. Stakeholders typically include Product users or operators, but always include the legal entity running the Program, too.

To this end, 

• a Portfolio represents a set of un-related Programs undertaken by the same legal entity. There is no Portfolio, if and where a legal entity is undertaking one Program only. In this case, all Portfolio-related management challenges may apply to this Program;
• a Project is a set of planned tasks that must be completed within a defined time and financial budget limit to deliver a defined and unique technical scope at a defined level of cost and quality. If and where a Program consists of only one Project, the Project can be regarded as a Program.