Ito Calculus
Ito Calculus is a branch of mathematics that extends the methods of calculus to stochastic processes. It is required to handle the random diffusion component of financial models because standard calculus cannot differentiate functions of Brownian motion.
The core of this field is Ito's Lemma, which provides a way to find the differential of a function of a stochastic process. It is indispensable for deriving the pricing formulas for complex financial derivatives and options.
Without Ito Calculus, we would be unable to mathematically describe how the price of an option changes as the underlying asset moves randomly. It allows analysts to work with continuous-time models that incorporate both deterministic drift and random noise.
This field is the bedrock of modern quantitative finance and algorithmic trading.