What you wrote is just t as a function of .
What that means, and how to best to describe it, depends on the specifics of the problem where that function arises.
Any of those is correct. Because is says one side = another, it is an "equation".
Because you can use it to find one value given another, it is a "formula".
Because you can think of a value of giving a value of t, it is a "function".
For your purposes, probably "formula" is best.