What is an academic and formal way to define variables?
The academic way to define variables is to simply describe them in the text. For instance,


  F = ma,
  where F is the force acting on the point mass m and a is the acceleration.


In fact, we want scientific texts to be readable, and not overburdened with unnecessary and obscure notations. Symbols have their role, but don't overuse them. Even more, a symbol like the triple bar has different technical meanings in different disciplines and using it in that way may just generate confusion.