Entering a national phase is a key procedure for enabling idea protection in selected countries. It is a later stage of patent prosecution, which is also related to the biggest cost of the patenting cost pie. Transition into national phase means taking all necessary steps to make a patent valid in a member state of some international patent-related treaty (PCT, EPC, etc.). It is not to be confused with national route, which means 'filing directly within a certain state', without going through PCT, EP or other regional patent applications.
In general all validation procedures need an assignment to a local patent attorney, who is capable representing applicant before the national authority. In most cases translations into national languages are required.
European Patent: validation and extension
The term 'validation' is most often referred to as making European Patent valid in a contracting state. It is done by translating the claims (sometimes full specification) and empowering a local attorney to present a validation forms for to the local intellectual property authority and to pay the official fees.
The term 'extension' is used to describe validation in the extension states, which are not Contracting States to the EPC but have instead signed extension agreements under which the protection conferred by European patent applications and patents is extended to the relevant country.
PCT: entering national phase
In case of PCT application, the transition is simply called 'entering the national phase'. PCT application can not be called a 'patent', because no grant is issued. Each contracting state of PCT may have individual set of prosecution rules, when an application is transferred from the international PCT phase, i.e. translation, additional search, examination, etc. However, some countries, like Belgium, Cyprus, France, Greece, Ireland, Italy, Latvia, Malta, Monaco, the Netherlands, and Slovenia have closed the route of validating a patent directly from the PCT international phase. Those countries might be accessed only through European Patent or purely by 'national route'.