TDL stands for Tally Definition Language
It is an programming language developed by Tally Solutions on which Tally Software works & business specific solutions can be developed.
Tally is not only an accounting application, but also a complete development environment which enables the software programmers to build extensions, customizations, add-ons, plugins and integrations using Tally Definition Language (TDL). TDL allows programmers to build a faster application without compromising on Speed, Reliability or data Integrity, which is core to Tally.
You can find ready to use TDL add-ons from here