The phrase "speed of light" is commonly used for the constant c =3E8 m/s, a feature that's "hardcoded" into the structure of spacetime. All massless waves and particles move at this speed, and it's a key concept in all fundamental theories in physics.
Light, in vacuum, is one fine example. But light in air or glass, or microwaves in a waveguide, move slower than c. The speed of actual light isn't the "speed of light". And of course there are plenty of other things besides electromagnetic quanta whose behavior is described by equations involving c.
The letter "c" alone isn't good for general writing and conversation, especially with non-scientists. It is fine as a mathematical symbol in equations, but isn't catchy or self-descriptive (albeit misleading) as "the speed of light".
EDIT: I have an impression I'm not asking this question in quite the right way. It's not at all about light, or phase or group velocities of light, but about our spacetime geometry having a fundamental constant speed "built in" - what to call it?
Historically it was discovered in connection with light, but c isn't specific to electromagnetism. It's important in all the fundamental forces, and in all situations with high energy particles flying about.
"Universal spacetime speed constant" would be an answer, but for that it's long and clumsy to repeat often in writings about physics. I'm looking for some name or phrase shorter and easier than that, but not so brief as just "c" which is okay in equations and tables of physical contants.