In economics, many empirical papers say that they utilized a difference-in-differences design.
I have already know the canonical DID method (two groups and two time periods) But, the setup in the papers includes multiple groups and periods. Also, they even just use a two-way fixed effects model. But, they use the term DID design. I am really confused from the use of the term because a two-way fixed effects model is not a generalized version of DID method.
Even in theoretical papers, researchers commonly use the term DID design. But, I cannot understand the exact meaning of DID design.
What is the exact meaning of the term DID design?