Well, It depends.
GEOGRAPHICALLY : Scandinavia refers to a the Scandinavian penninsula (Norway, Sweden, Denmark)
AS AN ETHNIC TERM : Scandinavian refers to anybody who lives or is from Scandinavia OR someone who dosen't live in Scandinavia but is a descendant of the Vikings (for example, a person who is mostly from Scotland who has documented proof of Viking ancestry is a Scandinavian)
NORDIC AS AN ETHNIC TERM
Nordic, as a Ethnic term may mean
- ///THIS PART IS DEBATED/// somebody who lives in one of the Nordic countries or their territories (Greenland, Faroe islands, Åland islands, Sápmi, Svalbard) regardless if they were born there or if they are immigrants from diferent countries (There is a debate over if people of diferent countries who live in Scandinavia are considered Scandinavians)
- Somebody who lives outside of Nordic countries or their territories but has registered Nordic ancestry, since Nordic countries do a great job promoting Genealogy and keeping records (Scandinavian-Americans, Scandinavian-Canadians etc) Most foundations helping Scandinavian-Americans consider a person who has quarter or more Scandinavian ancestry as a Scandinavian-American
- ///THIS PART IS DEBATED/// an Indigenous Nordic person, Such as a Greenlander, Kven, Ingrian Finns or Sámi regardless where they live. a lot of people consider these people to fall under Indigenous People instead of Ethnic Scandinavians. A person is considered Sámi if they have 50% Sámi ancestry, and a Kven if they had Finnish ancestors residing in Norway in the 1800s and later
AS A CULTURAL TERM : Usually, Scandinavia and Nordic is under the same term. There may be a Scandinavian festival that also has celebrations dedicated to Finland and Iceland. There are sometimes maps that only show Norway and Sweden as the Scandinavian penninsula due to the Mountain border. Nordic territories are usually not shown (espiecally Sápmi, or they just use the No-No L word for Sápmi.)
Hope this helps.