What is a general fact rule or principle expressed in mathematical symbols called?

Answer 1

If it yields a practical result, it's a fomula, such as Newton's Law of Gravity, or the period of a pendulum.

NLOG: f = (k.m1.m2)/d2 gives the gravitational attraction between two masses.

If it yields a pure number, with no actual dimensions or practical application, it's an equation or an identity.

y = x2 + 2x + 1 tells us that, for x = 1, y = 4, but we get no real-world usefulness.