What the X stands for in the front of Elliptic curve names like X25519 I have seen Curve25519 and X25519, Curve448 and X448. I've seen a small note in this answer

(Historical note: Originally, X25519 was called Curve25519, but now Curve25519 just means the elliptic curve and X25519 means the cryptosystem.)


Is it a standard to say CurveABC is the Elliptic Curve and XABC is the cryptosystem?
Or, it is just a convention between the Elliptic Curve Cryptographers.
It is an open standard by IETF.org
We can find the details in the mail archive of IETF, D. J. Bernstein's response;

It has become increasingly common for &quot;Curve25519&quot; to refer to an
elliptic curve, while the original paper defined &quot;Curve25519&quot; as an
X-coordinate DH system using that curve. &quot;Ed25519&quot; unambiguously refers
to an Edwards-coordinate signature system using that curve.
Kenny and others in Toronto recommended changing terminology to clearly
separate these three items. Let me suggest the following terminology:

&quot;X25519&quot; is the recommended Montgomery-X-coordinate DH function.
&quot;Ed25519&quot; is the recommended Edwards-coordinate signature system.
&quot;Curve25519&quot; is the underlying elliptic curve.

All relevant coordinate systems already have standard names in the
literature, and I would suggest sticking to those names whenever it's
necessary to discuss the coordinate systems per se: [...]

Bolds are mine.