Answer by FeedbackLooper for How would you call a function that doesn't have a formula?

You can say that there is "no closed form" expression for it. If you know that the function is uniquely defined by some property or equation you cannot solve explicitly, e.g. if your $f(x)$ satisfy something like:$$2^{f(x)}+3^{f(x)} = x$$which cannot be solved explicitly (using e.g. elementary functions, or a finite number of operations) for $f(x)$, you can say that the your function is "implicitly defined by" that property.