a dialect of Lua that adds a Lisp style syntax and removes some of the ambiguity that Lua has. Namely by providing airity checks, removing inline returns (the final value of a function is what is returned), using prefix notation to remove operator precendence, and more.
By running on Lua, fennel offers the portability, simplicity, and speed of lua with the extensibility of lisp with a robust macro system. There is almost 0 overhead using fennel over lua, and it can be embedded with a single executable and library file.
If you want to compare lua code to fennel code, check out the antifennel live demo