You have posted a lot of questions here without defining what your variables mean. The E=mc² equation is well known enough that most people probably understand that E is energy, m is mass, and c is the speed of light. However, you don't define terms in more obscure equations either. What's "pc", for example? Enough with the sloppy science!

All of them are common letters. Person who have completed Newtonian mechanics, all them will know what they are. What they represent.

I don't think I should define what F=ma is. Even E=$\dfrac{1}{2}$mv$^2$. All of them are common letters. But if there is any new equation or weird notation or not famous equation then I should explain. If you asked what $m$ represent then I would say that's a good question, but here what creates the question that is the understanding of $m$.

Didn't read about $pc$ in your comment earlier. I don’t know if you have reedited.

Anyway, p is common notation and you have said what c is in your comment. Finally, I think what let you question that is - there's no vector symbol. I haven’t seen anyone using the vector symbol earlier E=$\vec pc$ so one question raise if momentum is vector then why energy is not? Multiplication of vector with a non-vector returns vector.