nl is a file format for presenting and archiving mathematical programming problems.
[1] Initially, this format has been invented for connecting solvers to AMPL.
The nl format supports a wide range of problem types, among them: The nl format is low-level and is designed for compactness, not for readability.
Most commercial and academic solvers accept this format either directly or through special driver programs.
AMPL/MP library[5] contains an NL writer and SOL reader.