What is a brief history of the term "reformed" as in Reformed churches and Reformed theology?

Question

Why do theologies and churches based on the teaching of Calvin consistently self-identify using the term "Reformed" even down to the 21st century, such as the 2019 Reformed Systematic Theology by Joel R. Beeke and the newly created denomination in 2021 Alliance of Reformed Churches?

Why the early usage was not consistently "Calvinist" like how Lutherans kept using "Lutherans" even though over the centuries Lutheran theology diverged rather significantly from Martin Luther himself?

One Lutheran commenter said:

Sometimes reformation theology is called “reformed” theology. So, Lutherans would be included. But strictly speaking Calvinism is what you are thinking about. So, in that sense I get where you are coming from.

This got me thinking, how did the term "Reformed" start to be used to identify a theology or a church based on the teaching of Calvin?

I would like this question to be broadly suggestive as a historical question that explains the usage "Reformed" as a self-identifying label. But here are some angles that an answer may want to explain:

• When was the term started to be used? This is a cross-language "when", as the Dutch used Reformeerte or Gereformeerde (see here), the French used Réformée (see here), and the German used Reformierte (see here).
• Did linguistic / word history have anything to do with it?
• Did Calvin himself use the term "Reformed" to identify his theology?
• Which group started to use the term? Was it a self-identifying term, or was it a term used by other groups to label Calvinists (such as how the term "Anabaptist" got started)?
• Was it an early attempt to claim the whole Protestant reformation to Calvinism, thus denying Lutherans participation as a valid reformation theology?
• Was it a feature only in certain parts of Europe, associated with a certain language (like Dutch, German, English, French)?
• Why down to our century we still use "Reformed" in naming a theology rather than a more obvious "Calvinist", or more theologically precise "Baptist" or more ecclesiologically precise / "Presbyterian"? What motivated the 21st century groups to still prefer the term "Reformed" in this context?

Answer 1

The Encyclopedia Britannica article Reformed Church provides a short explanation on how (as the commenter said) the term "Reformed" was used by all Reformation churches, and how later on other groups like the Lutherans preferred another name:

Originally, all of the Reformation churches used this name (or the name Evangelical) to distinguish themselves from the “unreformed,” or unchanged, Roman Catholic church. After the great controversy among these churches over the Lord’s Supper (after 1529), the followers of Martin Luther began to use the name Lutheran as a specific name, and the name Reformed became associated with the Calvinistic churches (and also for a time with the Church of England). Eventually the name Presbyterian, which denotes the form of church polity used by most of the Reformed churches, was adopted by the Calvinistic churches of British background. The modern Reformed churches thus trace their origins to the Continental Calvinistic churches that retained the original designation.

Thus, it seems that the groups that use the name "Reformed" today still maintained the 16th century meaning:

1. Originally, to distinguish themselves from the "unreformed" or unchanged Roman Catholic church.
2. Shortly after, as the other reformation groups began to differentiate themselves theologically (such as the Lutherans after the 1529 disputation with Zwingli over the Real Presence of Christ in the Eucharist), the Calvinistic churches continued to use the term "Reformed" in the increasingly restricted sense theologically (as following Calvinism).

Note: As I come up with more citations and more details on various angles relevant to the history of the term "Reformed", I'll continue to revise this answer.