Definition

A process $(B_t)_{t\ge 0}$ is Brownian motion if:

  1. $B_0=0$
  2. It has independent increments
  3. For every $s
$$ B_t-B_s\sim N(0,t-s) $$
  1. Its sample paths are continuous almost surely

Interpretation

Brownian motion is the canonical model of continuous-time random motion with Gaussian increments and continuous paths.