AutoRegressive Distributed Lag is a time series model where the dependent variable is a function of its own lags, other variables, and their lags. ARDL is convenient for modelling I(0) and I(1) variables together and for cointegration testing.
ARDL stands for "Autoregressive-Distributed Lag". In its basic form, an ARDL regression model looks like this: $$ y_t = \beta_0 + \beta_1 y_{t-1} + \dots + \beta_p y_{t-p} + \alpha_0 x_t + \alpha_1 x_{t-1} + \dots + \alpha_q x_{t-q} + \varepsilon_t $$ where $\varepsilon_t$ is a random "disturbance" term.
The model is "autoregressive", in the sense that $y_t$ is "explained (in part) by lagged values of itself. It also has a "distributed lag" component, in the form of successive lags of the $x$ explanatory variable. Sometimes, the current value of $x_t$ itself is excluded from the distributed lag part of the model's structure.
ARDL models have come to play an important role recently in the modelling of non-stationary time-series data. In particular, they are used to implement the so-called "Bounds Tests" (Pesaran & Shin, 1999; and Pesaran et al., 2001), to see if long-run relationships are present when we have a group of time-series, some of which may be stationary, while others are not.
Based on Dave Giles' blog posts "ARDL Models - Part I" and "ARDL Models - Part II - Bounds Tests".
Pesaran, M. H. and Y. Shin, 1999. An autoregressive distributed lag modelling approach to cointegration analysis. Chapter 11 in S. Strom (ed.), Econometrics and Economic Theory in the 20th Century: The Ragnar Frisch Centennial Symposium. Cambridge University Press, Cambridge.
Pesaran, M. H., Shin, Y. and Smith, R. J., 2001. Bounds testing approaches to the analysis of level relationships. Journal of Applied Econometrics, 16, 289–326.