Like algebraic expressions are logarithmic, Experimental, trigonometric, differential, etc., expressions also there?
I am not referring to functions, but just expressions or equations. Are their definitions similar to how we define their corresponding function?
I know Functions are more frequently and widely used than any expression, why?
An expression is essentially a string of symbols (e.g., $$\frac{-b\pm\sqrt{b^2-4ac}}{2a}$$ or $x^2$) with some technical requirements; an equation is a relationship between expressions that is given by an equality (e.g., $c=2\pi r$).
I doubt that functions are more frequently/widely used than expressions. In mathematical literature, a typical author talks about the LHS or the RHS of the definition of a function (and the derivations thereof) far more often than the function itself. For every equation, there is at least two expressions.