dS = mu S dt + sigma S dW. Prices stay positive and are lognormally distributed.
The subtlety worth knowing. The drift of the log price is mu - sigma^2/2, not mu. That gap is volatility drag, and it is why the median outcome falls below the mean.
What it gets wrong: constant volatility (real volatility clusters), continuous paths (real prices jump), and thin tails. It remains the baseline because it is tractable, not because it is accurate.