What does using "Euclidean" before anything mean in Mathematics?

Disclaimer: I'm basically just summarizing what I found on the English Wikipedia page for "Euclidean" through the lens of my experience, biases, and understanding of the math.

It seems there are three main categories of usage of the word "Euclidean" meaning "related to the ancient Greek mathematician Euclid of Alexandria". Essentially all usage stems back to things he wrote about in The Elements, a collection of mathematical facts and justifications. There are:

1. Concepts related to his work on geometry.
2. Concepts related to a method he described for finding the greatest common factor of two number.
3. A couple assorted concepts tied to specific things from The Elements.

Geometry

In The Elements, Euclid described what we might think of as "standard" geometry for shapes in a flat plane or in three dimensions. This is called Euclidean geometry, especially when talking about the geometry of a flat plane, or the specific axioms and arguments Euclid presented in The Elements.

There are a bunch of related terms:

• A "Euclidean triangle" (a term used on the page for triangle groups) follows the rules of Euclidean geometry in that its angles add up to exactly $\pi$ radians ($180^{\circ}$) (as opposed to a triangle on a sphere). There are other similar terms to distinguish a situation from non-Euclidean geometry, like "Euclidean angle" (used on Ask Dr. Math).
• The Euclidean distance is the one we first learn about for points in $\mathbb R^n$, ultimately stemming from the Pythagorean theorem. It is closely related to the Euclidean norm for calculating the length of a vector.
• A Euclidean ball is a ball (set of points up to a given distance away from a center point) defined via the Euclidean distance.
• Euclidean space is usually something like $\mathbb R^n$ with the standard dot product for calculating angles and lengths (via the Euclidean distance). If it's not phrased as "Euclidean vector space", there may be certain restrictions like "distinguish between points and (Euclidean) vectors and don't choose which point is the origin".

and more.

Euclidean Algorithm

Something that doesn't need to be tied to geometry which appears in Euclid's Elements is the Euclidean algorithm for finding the greatest common factor/greatest common divisor ($\gcd$) of two numbers. This has a web of connections to other concepts in mathematics:

• It is extended to the extended Euclidean algorithm to find coefficients $x$ and $y$ satisfying $ax+by=\gcd(a,b)$.
• A Euclidean domain or "Euclidean ring" is an algebraic construct where the (extended) Euclidean algorithm still works, even if we're not dealing with usual numbers.
• A Euclidean domain's Euclidean function is the measure of size you need for division and greatest common divisors.
• And Euclidean division is the formal statement of how division works in a Euclidean domain (though Wikipedia doesn't have a source for that being the origin of the phrase). (Apparently, "the Euclidean division" could refer to a division of the Intermediate Math League of Eastern Massachusetts.)

Other Ideas

• Euclid's Lemma is a key fact about prime numbers covered in Euclid's Elements, which is essentially used to define a more general concept of "prime element" in abstract algebra.
• A Euclidean relation is one with a property resembling the first axiom from Euclid's Elements.
• This isn't about mathematics directly, but a city in Ohio was named after the mathematician Euclid, and Euclidean zoning can refer to a type of zoning used there.

Basically, Euclidean means "globally flat".

In the 1800s, non-Euclidean geometries were discovered. In these geometries, there is implicit curvature in the space (so they're non-flat).

Of course, by this time $R^n$ had been invented, and it was realized that these curvature calculations ended up with zero curvature on $R^n$ (so they're flat). It makes sense to generalize the term to $R^n$, even though most of what Euclid wrote applies to $R^2$ and $R^3$.

There are specific concepts that bear the name "euclidean" in a precise way.

For instance, a euclidean vector space is a real finite dimensional inner product vector space; a euclidean ring is a ring that satisfies some form of euclidean division.

Then there are "looser" concepts of euclideanity: when you say something is euclidean, depending on the context, it can refer to various things.

The euclidean plane, for instance, refers to $\mathbb R^2$, with more or less structure depending on the context (e.g. the euclidean metric, or norm, etc.)

Euclidean space refers to $\mathbb R^3$ or more generally $\mathbb R^n$, again with more or less structure

There's also euclidean geometry as was pointed out in the comments and in another answer, which corresponds to Euclid's axioms for geometry (as opposed to non-euclidean geometries); the euclidean algorithm (for finding a gcd); euclidean division (in the usual context of numbers or in the context of polynomials for instance).

I'm not sure there are that many more places where the term is used; but sometimes there will be a specific, precise, mathematical definition, and sometimes it'll just be in reference to something Euclid did, in a loose way.