Black-Derman-Toy

The Black-Derman-Toy model (BDT) is a popular short rate model used in the pricing of bond options, swaptions and other interest rate derivatives.
It is a one-factor model; that is, a single stochastic factor - the short rate - determines the future evolution of all interest rates.
It was the first model to combine the mean-reverting behaviour of the short rate with the lognormal distribution