What the X stands for in the front of Elliptic curve names like X25519
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.