Trignometry - tRi+koN+maatR(i) - Three + Angles + Measurement
The discovery of oscillating nature of Sin and Cos Ratios - tied with physical nature of cycling radius and oscillating horizontal and vertical points - was a giant step in understanding Nature.
The whole of Physics could not have made progress without it. All attempts of Unifying all Nature's Forces owes to this fact.
This was also an insight to the Rotary Nature of Universe.
The first modern documented use goes back to Aryabhata in Surya Siddhanta. To get a glimpse of this work, one can read Vedic Cosmogony.