nl is a file format for presenting and archiving mathematical programming

Mathematical Programming

Mathematical Programming, established in 1971, and published by Springer Science+Business Media, is the official scientific journal of the Mathematical Optimization Society. It currently consists of two series: A and B. The "A" series contains general publications. The "B" series focuses on topical...

 problems. It supports linear

Linear programming

Linear programming is a mathematical method for determining a way to achieve the best outcome in a given mathematical model for some list of requirements represented as linear relationships...

 and nonlinear

Nonlinear programming

In mathematics, nonlinear programming is the process of solving a system of equalities and inequalities, collectively termed constraints, over a set of unknown real variables, along with an objective function to be maximized or minimized, where some of the constraints or the objective function are...

 optimization problems as well as complementarity problems

Complementarity theory

A complementarity problem is a type of mathematical optimization problem. It is the problem of optimizing a function of two vector variables subject to certain requirements which include: that the inner product of the two variables must equal zero, i.e.  = 0...

 (MPECs), in discrete or continuous variables. Initially this format has been invented for connecting solvers to AMPL

AMPL

AMPL, an acronym for "A Mathematical Programming Language", is an algebraic modeling language for describing and solving high-complexity problems for large-scale mathematical computation AMPL, an acronym for "A Mathematical Programming Language", is an algebraic modeling language for describing and...

 but then it has been adopted by other systems such as COIN-OR

COIN-OR

COIN-OR, which stands for Computational Infrastructure for Operations Research, is a project that aims to "create for mathematical software what the open literature is for mathematical theory." The open literature provides the OR community with a peer-review process and an archive...

 as one of the input formats and FortSP

FortSP

FortSP is a software package for solving stochastic programming problems. It solves scenario-based SP problems with recourse as well as problems with chance constraints and integrated chance constraints...

 for interacting with external solvers.

The nl format is low-level and is designed for compactness not for readability. It has both binary and textual representation.
Many solvers such as CPLEX

CPLEX

IBM ILOG CPLEX Optimization Studio is an optimization software package. In 2004, the work on CPLEX earned the first ....

, Gurobi

Gurobi

Gurobi is a commercial software package for solving large-scale linear optimization, quadratic optimization, and mixed-integer optimization problems...

 and MOSEK

MOSEK

MOSEK is a software package for the solution of linear, mixed-integer linear, quadratic, mixed-integer quadratic, quadratically constraint, conic and convex nonlinear mathematical optimization problems. The emphasize in MOSEK is on solving large scale sparse problems. Particularly the...

accept this format either directly or through special driver programs.

The AMPL Solver Library (ASL) which allows to read the nl files and provides the automatic differentiation functionality is open-source. It is used in many solvers to implement AMPL connection.