Which speed of light constant do you use in E=mc²?

[ANO94]

i know this might be a dumb question but it was in my head and i would love to hear an answer,

so the famous E=mc2..e obviously energy, m mass, c is the speed of light,
but what speed of light? we all know that c is constant at vacuum but it changes when it goes through another medium like air or glass or whatever it is, so my question was when scientists actually apply this equation...what c do they use? like do they use c of air for example or d they only use the normal c which is 299,792,458 metres per second?

 

[Bloodthunder]

"c" always refers to "speed of light in a vacuum"
so there's no confusion here.