What exactly is a Schengen visa?

@user129387, in answer to your comment, the source for @GiacomoCatenazzi statement is the:

Schengen Border Code Article 6 (1)(b):

they are in possession of a valid visa, if required pursuant to Council Regulation (EC) No 539/2001, except where they hold a valid residence permit or a valid long-stay visa;

The referenced visa can have the type 'A' or 'C' (Article 2(5) and Article 2(2) of the Community Code on Visas).

The meaning of the letters used are defined in the Community Code on Visas ANNNEX VII Point 7 ‘TYPE OF VISA’, where 'long-stay visa' (also called elsewhere National visas) will have in the field type: 'D'.

Also the application forms themselfs use this terminology: Application for Schengen Visa for types 'A' and 'C' and Application for a national visa for type 'D'.

It depends on what you mean for "national visa". Schengen visa are standardized visas which are issued by Schengen countries. Countries should issues Schengen visa if possible.

But every country can issues national visas. Such visas can be linked to work or residence visas, diplomatic visas, but also some humanitarian visas or for special cases (some family visa, visa on partially recognized countries, etc.).

As an example, from a official site (Switzerland): the "Visa D" is only a national visa (see https://www.eda.admin.ch/eda/en/fdfa/entry-switzerland-residence/visa-requirements-application-form.html). If I remember correctly, Switzerland issues some national visa also for short official visit to UN in Geneva.

"Schengen visa" is a term that can mean different things in different contexts. Sometimes it is used to distinguish type C visas from other types of visas issued under the Schengen Visa Code. In that context, the term does not include type D visas. Sometimes it is used to denote any visa that will let you board a flight to the Schengen area. In that context, the term does include D visas.

There is a common but sorely mistaken belief that a word or phrase used in law must have a single unchanging definition in every context. For example, "state" can denote an entity such as Bavaria or Wyoming or Chiapas, but it can also denote an entity such as Germany or the USA or Mexico, and in fact this is what it usually means in international law. "Person" can include corporations in some contexts, but in others it does not. In US law, "foreign national" includes green card holders, except for campaign finance law, where it does not. Similarly, in the Schengen Borders Code, "third-country national" is defined explicitly to exclude third-country nationals who qualify as family members of EU citizens under EU free movement law. So a Japanese green card holder married to an Italian citizen is a "foreign national" in the US except for the purpose of donating to a political campaign, and she is a "third-country national" in the EU except for the purpose of crossing the external Schengen border -- but only if she is doing so to accompany or join her Italian-citizen spouse.

Context matters.

There are 4 types of visa issued in the Schengen area.

• A visa - Airport transit
• B visa - Transit with a stay of up to 5 days in the Schengen area, for travel through the Schengen area to the final destination. No longer available, has been superseded by a C visa with the purpose listed as "Transit".
• C visa - Short term visit
• D visa - Long term stay, employment, residence, etc...

A type C visa is usually generally referred to as a "Schengen visa" as it is governed by the Schengen code, found here (as are transit visas). It is probably relevant to highlight that the Harmonised form outlined within the Schengen code is titled "APPLICATION FOR SCHENGEN VISA" which I think clarifies why these visas are referred to as "Schengen visas".

Where as a "National visa" maybe issued as a D visa and while there may be some rules that are common across the Schengen area the countries are free to decide the procedures and the requirements for granting these visas and it is not covered by the Schengen code.

So yes they are different visas, a National visa is not a type of Schengen visa, but all visas issued in the Schengen area share the same alphabetical naming conventions.

I hope that answers your specific question.

The way most professionals (lawyers, border guards, etc.) would use the phrase, a “Schengen visa” is a visa issued under Regulation 810/2009 (the Schengen Visa Code). So it would include Schengen short-stay visas (ex-“type C”) and airport transit visas. Either can be valid across the area (uniform visas) or be restricted to some countries (visas with limited territorial validity). However, the phrase “Schengen visa” is not found or defined anywhere (that I know of), the regulation just calls it a “visa”, see article 2(2).

Confusingly, most national visas issued by Schengen countries now come in a very similar format with a big “D” where Schengen uniform visas have a “C”. Hence, the notion that national visas are “type D Schengen visas”. At the same time, their legal basis and consequences are unmistakenly different. The phrase used for those in Regulation 2016/399 (the Schengen Borders Code) is “long-stay visa”.

What you will find is that where it matters, e.g. in the regulations (Borders Code and Regulation 1683/95) and in decisions of the Court of Justice of the European Union, the only phrase used for a short-stay Schengen visa is “uniform visa”. Neither uniform visas nor national long-stay visas are called “Schengen visas” in this context. Any competent national authority who wants to write something legally sound should follow a similar rule and at least specify whether they mean a short-stay visa (but I wouldn't be surprised if you find me some official website with sloppy writing somewhere). Obviously, “uniform visa” isn't exactly obvious either and technically wouldn't cover LTV visas so you really cannot avoid long phrases and definitions if you want to be comprehensive.

Even the writers of the non-official websites you link to seem to be aware of the issue. I don't read them as implying that national visas are or aren't in the same category, quite the opposite. Both websites are quite carefully written, both go out of their way to distinguish national and short-stay visas, and both specify what they mean because they know they cannot rely on the phrase “Schengen visa” to be self-explanatory.