The Heath-Jarrow-Morton (HJM) model is a mathematical framework used in finance, particularly in the field of interest rate modeling. It was introduced by economists David Heath, Robert Jarrow, and Andrew Morton in a seminal paper published in 1992. The HJM model is part of the broader family of continuous-time interest rate models and is designed to capture the evolution of interest rates over time.

Key features of the Heath-Jarrow-Morton model include:

1. **Continuous-Time Framework:** The HJM model operates in continuous time, providing a more flexible and nuanced representation of interest rate movements compared to discrete-time models.

2. **Forward Rate Dynamics:** The model focuses on the dynamics of forward interest rates, which represent the future interest rates at different points in time. The forward rate is the interest rate agreed upon today for a loan or investment to be executed in the future.

3. **Stochastic Factors:** The HJM model incorporates stochastic (random) factors to account for uncertainty in interest rate movements. These factors are often modeled as stochastic processes, such as Brownian motion.

4. **No-Arbitrage Principle:** The model adheres to the no-arbitrage principle, ensuring that there are no risk-free arbitrage opportunities in the financial markets.

5. **Volatility Structure:** The model allows for a time-varying volatility structure, meaning that the volatility of interest rates can change over time. This feature helps capture the observed volatility smile or skewness in the implied volatilities of interest rate options.

6. **Term Structure of Interest Rates:** The HJM model is designed to capture the term structure of interest rates, providing a framework for understanding how short-term and long-term interest rates are related.

7. **Calibration:** Like many financial models, the HJM model requires calibration to market data. This involves adjusting the model parameters to match observed market prices of interest rate derivatives and other relevant financial instruments.

8. **Extensions and Variations:** Over the years, researchers have extended and modified the HJM model to incorporate additional features, such as jumps in interest rates or different types of stochastic processes.

The Heath-Jarrow-Morton model has been influential in the field of interest rate modeling and has contributed to the development of more advanced models. It is often used in conjunction with other models and techniques to analyze and manage interest rate risk in financial markets.